We haven't really done the halloween thing (except for one year when Madison decided that a little straw cowboy hat was HERS and no one else could touch it (pictures are somewhere on the computer), but for the past two years we've gone to a greyhound picnic that has a costume contest.
Last year we only had Madison and it was olympic time (woo!) so we dressed her as an olympian (toga, toga, toga!) with the olive branch head dress and everything.
This year we had (it was actually yesterday) Madison AND Moonshine... the theme was nautical so we dressed Madison as a Mermaid (get it? the movie Splash!) and Moonshine as a sailor (pics to come!)...
The sailor outfit was great... but like everyone has said, I'd just go buy a big little kids outfit and make some changes so it will fit. Spencers (usually in a mall) has dog costumes, but they just go over the necks... nothing special...
I think this year, Mads and Moo may go as Ragedy Anne and Andy (so we can re-use the sailor costume)...