How do I change the heating element on my 50 gallon electric Reliance 501 hot water heater? Where can I get the heating elements?

Buy elements at local hardware store or home center, like Lowes
Look at label on side of tank.
Label shows element wattage.
Element is also printed on end of element in tiny letters-numbers.
Typical elements are 4500 and 5500 watts.
Tank needs 10 gauge wire and 30 amp breaker for 5500 watt element.

Water heaters are fully repairable by homeowner.

1) Have you tested water heater to see if element is problem? You need cheap multi-meter from Home Depot.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

2) To change element
a) turn off power
b) drain water from drain valve ... once it starts draining, loosen element slightly while tank still has weight to hold against you turning element wrench.
Here's what to do if water heater won't drain:
http://waterheatertimer.org/Water-heater-will-not-drain.html
c) open covers on tank, remove element using element-wrench from hardware store or home center.
If element won't come out:
http://waterheatertimer.org/How-to-remove-old-water-heater-element.html
d) Clean out tank if needed.
e) Replace element.
f) Fill tank entirely with water before turning on power or element will burn out.
g) Once tank is filled, and water flows steady out of tub spout, turn on power.
Each steps above is shown on following link:
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html

3) You need to buy some things:
Screw-in type (not bolt-in type) Element that matches wattage on tank label. New Element comes with washer.
Inexpensive Element wrench.
Inexpensive multimeter if you are going to test parts first.
Some pieces of pipe and hose if tank sediment needs to be cleaned out.
Washing machine hose if tank will not drain (return after job is done).

4) Also:
You need flat tip and phillips screwdriver.
Lighting to work on tank
Some WD-40 if element is stuck in place.
Garden hose to drain tank.
Shop vac and duct tape if you need to vacuum out sediment.
Tarp to protect floor and carpet
Maybe a knife to cut back insulation on wire if wire is burned at element.

Web links above go through entire process step by step.
Add a comment if you need more help, I'll be back around 5 CST