31s is the max for un-lifted 2WDs
Get on tirerack.com and compare your options and get a price range. DO NOT BUY FROM THEM. They are hella expensive.
You also need to look into new wheels, running big tires on the stock 15x7s usually don't look that great. But since you would only be doing 31s it wouldn't be that bad.
If you give me a price range I can tell you what you should be looking at for tires.
Don't forget to check your local Craigslist as well.