The plastic threaded base for the jar on this blender haier 2 speed / glass jar is cracked - is part available ? thanks

I'd be tempted to try the smallest amount of olive oil, applied with a paper towel to both surfaced worked into threads. You need very very little. Alternatively get some different jars!
You can get food safe grease but it's very expensive and usually used industrially

It will either be the rubber gasket (it goes between the glass jar and the blade assy) or it could be a worn bearing/bush around the chopping blade shaft. Replace either the gasket or the blade assy.

A couple of things you can check.
Unscrew the glass jar from the bottom.
Check to see if the grey seal is between the metal blade assembly and the glass jar. I had this issue and it turned out to be that a friend had reassembled the blender jar parts after taking it apart to clean it.

Are you sure you are installing the gasket correctly?

Leaks are usually the result of a misaligned or cross-threaded blender jar base or from the gasket being installed incorrectly. Make sure the gasket is installed between the blade assembly and the screw on base.

If this does not help, please reply or repost.

remove the screw on blade holder, and make sure the gasket is in place ,, check the seal on it,, if its defective replace it,, check along the threads to make sure that the canister isn't cracked

Make sure the blade assembly is put together in the correct order. The blade needs to go through the gasket so that the order is pitcher glass, gasket, and metal blade base. Then the final plastic cap threaded onto the pitcher. If the gasket is not between the pitcher and the metal blade assembly, you will have leaks.
Also, check the gasket to be sure it's not cracked and that it doesn't have residue on it.
I hope this helps. If it does, please come back and rate my suggestion!

Yes the white plastic base comes off. There are several model of the BL10450; here are the Instructions which contains an illustration of the assembly:

https://www.unitycart.com/pjss/parts/parts_classic.asp

See the notes on Blender Jar Assembly. It turns on clockwise; off would be counter clockwise.

I did the same thing. Order parts at thegourmetdepotco.com -- This problem is caused when you put the rubber gasket underneath the blade instead of above it. This causes two problems: 1. It lifts the teeth on the jar away from the teeth on the drive gear, which can grind down and break off the teeth, and 2. If the drive gear contacts the rubber gasket, it will melt the rim of the drive gear. In the future, to reassemble the jar after cleaning, first put the rubber gasket on the jar, then the blade, lastly screw on the base ring.

I will buy the blade set from you? I will return....

The drive is replacable and available from Cuisinart, you will have to sign a waver to get the part. the drive is screwed onto the motor shaft with reverse threads. the plastic drive will have to be completely broken off of the unit to expose a brass"nut" which was internal to the drive. depending upon your model you may have to remove the lower half of the base unit to access the lower part of the motor shaft. there will be a slot in the end of the shaft for a large flat blade screwdriver. this is used to hold the shaft still while turning the drive "nut" with a vice grip pliar or similar. the new drive is just threaded onto the shaft and normal operation tightens it.