The infant Stitch costumes were not available at the stores - only online and now they are sold out. I bought the 12 month size for my ds (who is only 5 months now) since it said it was for 6-12 months. It is a bit big (he is only 15 lbs - not a big baby) but we will have to make do. So if you do find one, make sure you get the right size as it would be a shame if it did not fit.
I would generally suggest ebay as a good source but it appears these costumes are going for $50-125 at this pont. WOW!!!! You might just keep looking on ebay for new listing though as a few have come up with Buy It Now prices of only $25 or so.
Good luck and I sure hope you find your costume!