floating reamer holders that fit in the tailstock. MSC Industrial Supply Co. http://www.mscdirect.comshould have them in stock. Heck, it is worth getting their catalog for any industrial supplies. Get enough of the catalogs and you can armor your walls, the catalogs are that big.
Clark, Clymer sells a floating reamer holder like JimV mentioned just for holding reamers. I use one when I can but a lot of smiths hold the reamer with a tap wrench. The tap wrench is either held by the left hand or allowed to rest on the compound rest. No matter how you do it the most important thing is that the bore is running true. The easiest way to get it running true is to use a barrel centering bar and your dial indicator. Brownells http://www.brownells.com sells the bar for around $65. They also sell the floating holder. George