Get down to the LFS
(local fish shop) and see what they have. Its better to get one rated more than what your tank is so if your tank is 20G get a filter that can handle 30 to 40G. Going one up is always good thinking in saltwater tanks.
If you go down the live rock route you wont need a filter. About a pound per galon of live rock is generally the rule. If you can get the live rock at a good price go for that as it is much better than mechanical filters. The addition of a protein skimmer would be good too.
If you decide to buy live rock make sure its cured first. I'f its not cured you could introduce polutants into your aquarium. Sometimes people sell fully cured live rock on 2nd hand webites. that would be the cheapest way to get some. make sure you check out their set up first.