I'm in zone 6B and get all my fruit trees (including plum) from Schlabach's Nursery (Medina, NY):
Schlabach's Nursery
2784 Murdock Rd, Medina NY 14103
phone: (585) 798-6198 (best time to call is AM)
Toll-free voice mail for catalog requests only: 1-866-600-5203.
They are an Amish nursery, but have great prices and ship very healthy bare root trees using good rootstock. Give them a call at 585-798-6198 to request a catalog. They sell both Japanese and European plum trees @ $16 per tree plus shipping. I've been ordering from them for the past 7 years without issue. The only catch is since their Amish, they have no webpage and all payments must be done by check or cash. Not really a big deal, but I thought I'd mention it.
Personally I'm growing a Green Gage Plum tree which is self-fertile (make sure you check on this as many plum trees need a cross pollinator).