EDIT: Birdhunter, as an after thought I was afraid my lighthearted jab below may be taken offensively. I have not deleted it as it is only intended in fun. I did want to take this oportunity to forwarn you though so as not to cause hard feelings. Actually perhaps a "sticky-ed" post with this sort of information (sources) would be helpful to all. Frankly though, right here is about the most helpful place I've found for freely traded information on this subject of converting side by side guns to rifle. Information that is equally helpful in working on shotguns as well.

As for books, Ellis Brown's "Building Double Rifles on Shotgun Actions" is unquestionably the best place to start asside from right here at NE.com. Not the absolute difinitive work but about as close as it comes. In reality Mr. Brown himself advises that his book is intended as "a guide" for those who would like to take on such projects. Some of his opinions and methods are somewhat biased (as he also admits) but you'll be hard pressed to find a similar body of information on the subject.

Parts suppliers? I thought you've done this before? Try the usual suspects; Brownells, NECG, Galazan, Jeff's Outfitters and don't overlook the many possibilities at Gun Parts Corp. Just Google "Gun Parts" and the list of potential suppliers is impressive. I've also picked up the odd lot of scarce parts on AuctionArm.com and Gunbroker from time to time. For gun specific parts you'll need to do a bit more work. Then of course many of the parts intrinsic to such firearms, particularly old and/or obsolete ones simply are not available commercially but must be bench made. The road aint smooth but it is scenic and interesting!