My opinion on this is just off the cuff. I have a few young Welsummers and a 7 month old rooster that was given to me, but I have been doing quite a bit of research getting ready to get more of my own.
1) I think there is a decent demand for them, although I think they get overlooked a little for the Marans that have a little darker egg color. I think in part they are rare because of the relative difficulty to find high quality breeders, and because there are conflicting reports regarding their egg production. I don't think they have the same demand as other dark egg layers at this time.
2) I have read a lot of good reviews of Whitmore Farms. If you are going for pet quality, he would be more than adequate, as I have read that he has some pretty good lines. He is one of the only breeders I have found that ships chicks. There are a few hatcheries that carry Welsummers, but they sell out quite quickly, and they are more expensive as well. If you are looking for show quality, there are various breeders that have some pretty high quality birds. I have read mixed reviews on if Whitmore is show quality, but I believe they are pretty solid lines. There were 5 original breeders who helped get the breed recognized by the APA and you can find descendants of their original flocks still. A couple of those breeders are still breeding and will ship hatching eggs.
I am assuming that you want the LF variety and not the bantam?
I sold both Welsummer and BCM chicks this year and both are extremely popular here in PA. I sold more Wellies because of being able to sex the chicks, but the BCM's were more profitable for exactly that reason (I sold them all of the same price, but the Wellie cockerels were given away or culled, whereas most of the BCM cockerels were sold as straight-run chicks). I sold every Wellie pullet I was able to hatch, but had to stop setting BCM's because the demand dropped off. I really have no idea why anyone would buy BCM chicks if they could get Wellie pullets for the same price, but some people did! I don't care that much for BCM's and would sell them all off except I like being able to supply them to local buyers because they are just not available anywhere else. Good quality (non-hatchery) Wellies are just as rare around here.
My stock of both breeds came from Whitmore Farms. I also have their BBS Ameraucanas. I think all are decent quality and value, and would recommend buying from them because they ship chicks and are far better than any commercial hatchery. They seem to be representative for their breeds, but I don't show and haven't even read the SOP for these breeds. None of these 3 breeds have been stellar layers (our Rhodebars and "barnyard mixes" lay about twice as well), but they pay their way because of their popularity as chicks.