Hello.  I have a 1990 Pontiac 6000 SE AWD and the body side mouldings are brittle and falling apart.  Any ideas on how i can get replacements?  

Hi David - welcome to FP.

The Pontiac 6000 is not supported very well by the aftermarket, I'm sure you know that. 

For cars that have little to no aftermarket / reproduction support, you have to rely on someone having what is called New Old Stock (NOS) parts - generally these are parts that GM produced as replacement parts back in the day (they are new - never used but old age-wse stock) or you have to purchased used/salvage parts from a donor car that is in good condition and you may have to do some re-conditioning yourself on them once you get them.

So general sources to look for NOS parts are:

eBay, Craigslist, Kijiji (Canada), local swap meets (not any of those going on right now sadly) and may be Amazon (sometimes you get lucky). There may be others but those are the major sources I would try first.

For donors, you have to check salvage/wrecking yards. You should check those in your local area if they have an 6000s and hope they are in good shape. Otherwise I have a list of wrecking yards I can recommend - they are generally in the southwest. However, I have no idea if they will have any 6000 parts since generally speaking, it is not a popular car for parts these days. These yards would probably have either the large cars or muscle cars from the 50s-70s in a lot of cases. That said, I will be happy to give a list if you are interested. 

Another good source for parts is:  car-parts.com  That's where I shop for Pontiac 6000 if my own mechanics shops can't find the parts.