Any idea where I can find a replacement for the "on" tripper?

oil the on switch

if it won't trip it, the on pin is not lining up right and u need to take it off and put it making sure it is right. if that's not it u got a bad timer

Sounds like the OFF tripper is worn down.
Timer needs to be replaced unless you have Intermatic-brand timer.

Intermatic is only company that sells plastic replacement trippers for their timers.
Except Intermatic on-off trippers are typically red-green.
It's possible your trippers faded in color.
Open following links to identify Intermatic timer and tripper:
http://waterheatertimer.org/Intermatic-timers-and-manuals.html#appliance
http://waterheatertimer.org/Intermatic-trippers-and-parts.html#trippers

Most time-all timers have two different trippers on dial.
Push down and rotate tripper to desired ON time and OFF time.
Then manually rotate dial until current time lines up with a mark or arrowhead.

To replace timer with new one:
Buy TN311c with 3 sets of on-off trippers that push onto dial, and 3-prong outlet.
http://www.amazon.com/Intermatic-TN311C-Heavy-Grounded-Timer/dp/B00002N5FO

Buy TN111c with 2 sets of on-off trippers, and 2-prong outlet
http://www.amazon.com/Intermatic-Appliance-Security-TN111C70-Settings/dp/B00002N5FI/ref=pd_cp_hi_3

If zones are not advancing, then 1) Clock motor is not rotating dial > replace clock motor, 2) on-off trippers are worn down > replace trippers, 3) or the day skipper is set to skip all days and all skipper pins are pushed down, 4) or sprinkler valve is bad.

The sprinkler timer dial rotates each 24 hours. The trippers cause sprinkler head to turn on zone 1 for example. Zone 1 waters until the dial rotates to part of dial with no tripper. Without a tripper on the dial, the timer turns off circuit to sprinkler. At this point, the sprinkler valve mechanically advances to zone 2 and waits for timer to turn off again. The timer does not advance sprinkler to next zone > the sprinkler valve is designed to advance each time circuit turns off. The timer just turns on and off. When next tripper rotates around, the timer turn on the circuit and zone 2 begins to water.
http://waterheatertimer.org/pdf/How-to-Set-and-Troubleshoot-Intermatic-sprinkler-timer.pdf

The silver pin inserted at 12midnight position is for advancing the day skipper wheel.
Open following link for parts and resources:
http://waterheatertimer.org/Intermatic-trippers-and-parts.html#sprinkler

The red tripper is ON.

Put trippers into dial, and then manually rotate dial to see how the lights turn on-off.
Rotate the round override switch and then rotate the dial again to see what affect the override switch has.
These actions will help you understand how to set power pack.

For replacement trippers and money-saving tips and useful troubleshoot information:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.

You can replace the 133T1115A switch if that's the suspect part:
http://waterheatertimer.org/Intermatic-trippers-and-parts.html#sprinkler

Other than the switch, the only other thing affecting timer would be the trippers.

Paragon 4000 series timers uses X2772 tripper
http://waterheatertimer.org/Paragon-timers-and-manuals.html

Google search x2772 tripper
Frontier Lighting shows they are out.
A couple other places show 56-60 dollars for the tripper

In November 2010, I got e-mail from Paragon saying that Paragon stopped making timers and parts for timers, except for defrost timers. 4001 is not defrost timer.

Intermatic has cross-reference link if you want to buy new timer:
http://www.intermatic.com/customer%20service/competitive%20cross%20reference.aspx
Cross reference shows T-101 timer matches your timer.

Aplus supply sells T101 for $60
http://www.aplussupply.com/intermatic/timer.htm
Amazon sells T101 for 43$ with free shipping over 25$
http://www.amazon.com/Intermatic-T101-Hour-Dial-Timer/dp/B00002N5FN

T101 wiring:
http://waterheatertimer.org/How-to-wire-T104-Intermatic-timer.html#T101

Yes, the T101M 120Volt pool timer requires both ON and OFF trippers:

Grainger sells T100 series on-off trippers:
http://www.grainger.com/Grainger/INTERMATIC-Timer-Tripper-2E054?Pid=search
Amazon sells T100 trippers
http://www.amazon.com/dp/B003YO2C5U/?tag=picc-20

Grainger sells the T100 125V clock motor
http://www.grainger.com/Grainger/timer-accessories/timers/electrical/ecatalog/N-8hi

How to wire T-101 120Volt timer
http://waterheatertimer.org/How-to-wire-T104-Intermatic-timer.html#T101

How Intermatic dial timer clock works.
Clock motor rotates dial.
Intermatic clock motor, located on back of mechanism, are replaceable
Trippers located on outer edge of dial push on-off lever
ON tripper pushes lever one direction
OFF tripper pushes lever opposite direction.
Lever pushes electrical contacts together and pulls contacts apart.
When lever moves, manual override also clicks back and forth with audible click.
To replace timer, replacement snaps into box without needing to changing box.

You probably need new 156T1978A tripper.
Trippers are made of softer metal and are designed to wear down and be replaced.
http://www.grainger.com/Grainger/INTERMATIC-Timer-Tripper-2E054?Pid=search

Open following link for troubleshoot checklist
http://waterheatertimer.org/How-to-troubleshoot-Intermatic-timer.html