generally you can watch for sales at bestbuy and circuit cities. avoid the low end models and brands at each store, eg dual. most of the kenwood, alpine, pioneer, fosgate, kicker, infiniti interior speakers are decent for entry level/stock replacement.
you can also look for special sales at audio stores like sound advice, tweeters, car toys, mobileone, etc. they usually sell higher end brands. i doubt you will find anything new and legit for under $100, but if you decided to spend a little more per set (left and right side for a pair): JL audio, Diamond, Focal, and MB Quarts are a good starting point for entry-quality.
i have bought some speakers from etronics.com before, they are similar to best buys and circuit city. only problem is the shipping if you dont like an item.