Im looking to upgrade the exhaust on my 99 grand am gt, im upgrading to a 2.5" exhaust setup from the stock 2.25" setup now, but im having difficulties finding a cat that would fit and has the same size piping for my car that i want to upgrade to. Should i get a cat with 2.25" pipes and weld it to the 2.5" setup im doing or os there a cat out there for my car that has 2.5" pipes? Any help and/or advice would be greatly appreciated

Sounds like your building your own.  You can use your original (if its in good shape) and get an adaptor from any auto parts store. Less than 5 ducks.  Or many places sell universal CAT's in your size.

...Less than 5 ducks...

 

 

 

Are you installing headers too or just the catback?  If it's just the catback, I say just get an adapter and keep the stock cat.  I have the SLP catback on my GAGT which is 2.5" and I've only ever had the stock cat on it and now a Cali carb replacement one.  I've had the SLP for about 8 years now with a stock sized cat and she still sounds wonderfully mean.  She sounds similar to a V8 and the GAGT actually purrs louder than my G8 GXP with Solo axlebacks for now. 

SEE, Cat's N Ducks can get along.  Who knew?