As I've noted before, the System 3 cartridge filter is a really, really crappy filter.

You can convert -- I converted the ones I was working with to DE -- but you essentially have to get the DE or sand filter manual, and buy everything shown EXCEPT the case.

You might try this company:
http://www.a1poolparts.com/
Lonnie 888-841-5050 x714
ljunghans AT a1poolparts DOT com
I'm ordering some stuff from them (for my local work) and was told they would be happy to sell parts to folks from here. The prices on the items I checked were lower than my 'wholesale' cost here.

Let me know how it goes.

PoolDoc

PS. You are MUCH closer to them than I -- you might want to check their physical address, and buy over the counter.