the twin carbs are supposed to have better topend while the single has great lowend. it also is a lot less hassle to maintain, since you only have one to jet and one filter to clean, plus there is no synchronizing ever needed. you could try the trinity website Trinity, and i think graydon makes one as well. those are for new ones. if youd like to go the used route try eBay or the for sale section in the forums.
Hope this helps, Cole