I bought mine from a local grower from Edmonton so not much help to you in finding C. percivaliana 'Summit' from a grower. Possibly Hausermann's as they seem to have some oldie but goodies.
Some sheaths do dry up early before the buds are developed. I don't know what kind of spots you mean, but I think I have had the odd spot on a dying sheath. So long as it isn't all wet and mushy, I would have been tempted to slit it open and possibly remove it or just keep an eye on the buds. I have Chia Lin 'New City' Another from Paramount that is supposed to be bloom size, but only marginally so. It has a recent large new growth, but still no sign of a sheath