You can use the stock belt.

I just changed to a 3.4 SC pulley, kept my stock belt because it was new also and bought a 3.5 tensioner pulley to replace the stock 3.0 pulley.
This combo works perfectly. I bought a Dayco pulley P/N 89006 (IIRC) to replace the the stock size.
I don't know if you'll need to do the same but it can't hurt to have the larger pulley on hand in case you need it. If you don't need it then you can always return it.

Kevin, read page 1 of this thread. Good luck!

I wanna up my boost a little

what is the smallest pulley i can run bone stock?

What is the smallest pulley i can run with colder thermostat and colder plugs?

What do you recommend?

I would not run ANY smaller size pulley with zero supporting mods. Many of our cars, mine included, have a few degrees of KR in stock form even. No pulley drop is recommended, at least until you can scan for KR and confirm you have none to begin with. With no KR and no supporting mods, you can likely go to a 3.6" pulley w/o issue. But being able to scan for KR is very important.

Assume you're referring to your "new" Riviera? Always helps to give us some basic car info when asking for help. You can easily add this info to your default signature.

With just colder plugs and tstat, I'd still not go lower than a 3.6 pulley unless you can confirm you have zero KR.

I didn't dare install a smaller pulley until I had installed Headers, 1.9 rockers, AL 104 plugs, 180* thermostat with PCM programming for the thermostat and fan settings first. Also gutted the airbox.
The plan was to open up the inlet somewhat by gutting the air box and putting in a high flow air filter then open exhaust with headers and high flow Cat, then the 1.9 rockers and new springs to open the valves a little earlier and close a little later. Then scan, tune, install 3.4" pulley and scan, tune some more to eliminate KR. I also put in cleaned and flow matched injectors, new wires, etc.
I did not want to sacrifice a good engine by installing a smaller pulley and increasing boost first without doing all the above.

i know this may sound like a dumb question. where can i find a 3.6" supercharger pulley for my 1996 Riviera?

if i replace the stock pulley with a 3.6" pulley, will i need to get a smaller belt?

look at the top of page 8 with a answer from turtleman.
some use a larger tensioner pulley to compensate for the small belt size increase.

You won't need to change the tensioner pulley with a 3.6" SC pulley. Just keep the belt and all other pulleys the same when running the 3.6". See page 1 of this thread.

Your current SC tensioner pulley is a 3.0" pulley.
When i went to a 3.4" sc pulley I changed the tensioner pulley to a 3.5" with stock length belt.
If after you have installed the 3.6" SC pulley you decide you want better belt wrap or are experiencing any belt slippage you can buy a 3.25" tensioner pulley for it. Which should still keep the tensioner in its normal operating range.

Just wondering if anyone had a smaller supercharger pulley for sale ? My car is basically stock so i didnt know how small i could go and be safe not hurting the engine. Let me know ! Thanks

Dont drop pulley sizes until you open up the exhaust a bit and have some way of scanning for knock. You could probably get away with a 3.6" pulley but it's still a risk.