Yea, you're best off either converting a roadbike to fixie... I'm actually working on a project myself... I just picked up an old Spalding Blade at the local Good Will thrift shop for $20 (You should def tune up the rest of the bike while you're at it. give it a full rebuild)
Performance sells a kit that changes your regular derailleur into a single gear set up. Have not used it, but have seen people who have and wouldn't mind trying it myself. Fixed gear and single gear are similar, but very different.