For inexpensive party tops, I usually favor Forever21 (which ships for free with any $75 purchase) or H&M. If you want something more flashy and unique, go to Bebe or Guess for some stylish tops.
Try places like Forever 21, Charlotte Russe and the BP department of Nordstroms. You can also find nice tops at places like H&M ,Macys, Wet Seal, or Anthropologie, sometimes even JC Pennys has cute tops.
Forever 21 has a lot of great clothes at really lowe prices, they are all over the place so it shouldn't be hard to find a store. Make sure you love the top though, they have a no returns policy.