I have a 1997 SSEi and i was curious to know, what sized SC pulley I should get, and What other mods are needed, What is the highest pulley I can run with the littlest mods needed?

Take a look here, this should help you get started.
https://www.gmforum.com/performance-brainstorming-tuning-96/what-order-should-i-mod-my-engine-278662/

first of all, make sure your car is in tip top shape maintenance wise, fuel filter, plugs, wires etc. then scan fpr KR and after any KR has been dealt with, then get a new pulley. Typically a fenderwell intake plus plugs maybe a high flow cat/plog can run a 3.5-3.6 after tuning fine, then more mods equally a smaller pulley!

Hmm, thats more information ive heard/seen so far

Is there any good sites/vendors that deal with 3800s for a decent price?

First off, you will save a lot of money if you do your research and ask around before ordering parts, just a friendly warning from someone who has ordered their fair share of unnecessary parts.

I commonly use zzperformance, 3800performance, wbodystore, and a few others for specific parts. We also have a discount code for rockauto which has a lot of parts for our cars. Shop around and don't be afraid to ask questions if your unsure.

Im just curious to know about what all i should definitely buy for a smaller pully, im not wanting to go extreme, but id like to make it a little faster

The thread I posted above is the best guide I've seen as far as modifying your engine goes. Start with all the normal maintenance, then you want to open up the exhaust and air intake. That means gutting your air box, or getting some of cold air intake, free flow cat, resonator, mufflers, powerlog. Get a gauge that will read KR, a lot of people get the aeroforce scan gauges because they will read just about everything including allowing you to check and reset check engine lights. Doing all that should allow you to run a 3.6 - 3.5 pulley with no problems.

What exactly is a powerlog? And KRs?
Im a noob when it comes to forced induction and 3800s

A powerlog is a replacement for the front exhaust manifold that is less restrictive as far as letting air pass through it. This will reduce KR, which is Knock Retard. Knock Retard is a safety mechanism built into the engine which retards the timing when it detects engine knock to protect it. This can be caused by running to much boost. The more boost, the more heat. More heat = higher chance of predetination = knock = knock retard = slower car and possibility of damage to engine.

tyvm
So these are the bare necessities? How much of a gain am I looking at?