We hired the Playboys (or the Playboy Band) - www.theplayboyband.com . We hired them because my husband and I are both Ukrainian, and one of my dad's friends knows someone who knows the band (you know how that goes!). As much as their name (and the music that comes on on their website) might scare you off a bit at first, they're actually a really great band. They definitely play Ukrainian music, but they do great covers of country and rock and stuff. A couple of other bands that we were considering are UB and Millenia. Both these bands are a bit younger, and again, play ukrainian music along with lots of other covers. They play tons of weddings, hall parties, stuff like that. Especially for UB and Millenia you should be able to find videos on youtube and myspace pages. (use the band name and the word Ukrainian and you should be able to find them).
Click on each tool to see a sample.