I have an inline "brick" type supply with a detachable cord that I got on Ebay a while back, shortly after I got my mini. It's not really smaller than the OEM wall-wart, but it does fit easily into my Targus netbook bag along with some other stuff. Dimensions are about 3-1/2" x 1-1/4" x 1".
I can't point you to the exact vendor since it was so long ago, but since the Mini uses a standard 19V input there are plenty of adapters available. Of course quality will vary widely, the one I got has been good but there are a lot of crappy ones out there.