I got some problems with my OEM cat and wanted to put a high flow or performace cat on it but don't quite know where to get it. any ideas?

Covered for 80k miles under warrenty.

You can try the RP cat butt... there have been alot of failures.

http://www.rx7.com/store/rx8/rx8exhaust.html

what kind of failures?

studdering problem, checked everything and thats the last thing it could be but dont know if it falls under warranty cuz i bought the car from a non Mazda dealer used. its a 2004.

the cat warranty I think is a USA thing, not a mazda thing. If the cat is bad and you are not over 80,000 then you are covered and can get a new one.

i took it to the dealer and they said its not the cat so im kinda out of ideas. they dont even know whats causing the power loss.

loss of power is usuallly caused by a few different things so you really need to approach this with a 'process of elimination' mentality

Clogged Cat? : check
Ignition Coils: How old are your coils? ever replaced them?
Spark Plugs: how old are they?
Are you getting a CEL of any kind or even the ocassional blinking CEL?

if you've ruled out the CAT as a problem and it's not ignition related then it could be a compression issue. The only way to be certain is to have a compression test done.
your engine is warrantied 8 years, 100,000 miles.

EPA Emissions Warranty LINK (http://www.epa.gov/oms/consumer/warr95fs.txt)

Sounds like as Jedi mentioned, it's time for a compression test.

replaced ignition coils a week ago, spark plugs about 6 months, and i got the CEL P0410 but they didnt find anything wrong with that either. only told me to bring it back if the light comes on again.