I use decals quite a bit and while I can't help on where to get them in the US I have a few tips which may help.
I get digital decals (cheaper for small runs and perfectly good enough for my needs) and it seems there are two types - Cuccolini and Zimmer.
On the Cuccolini sheets the flux is all over the sheet, and I find that for an irregular shape that I cut out square, if the pot is made of stoneware I get a 'flux outline' (i.e. a square shape round the image) - however they usually work fine on earthenware as the outline generally disappears completely (although sometimes there is a very faint flux outline even on earthenware, only visible when you hold the pot up to the light).
On stoneware I use Zimmer decals (unless applying a square image where there is no flux outline problem). The flux is actually in the ink - so no outline problem. I have found that the Zimmer decals I use tend to get more blow-out holes from tiny air bubbles - this could be because the ones I use are much thicker (the Cuccolini ones are really thin once they have been soaked off in water) - or it could be that my stoneware glazes are a bit less smooth. Usually they work OK and I only really get problems on large areas.
Generally I would avoid doing huge areas of solid colour as you are more likely to get blow-outs.
To apply, I use a bowl of hot water to wet and warm the pots (usually mugs) and a bowl of cold water to soak the decals. Having the mugs wet means I can slide the decals around. I use a dry cloth to get any wrinkles or air bubbles out - I do get failures (this is pottery after all!) but generally it works well.
Hope this helps. Lots of my decals on my site at www.cavingpottery.co.uk