The late Hal Bruce was the plant taxonomist at Winterthur. The plant is:

HAL BRUCE Parentage unknown; elepidote
8ft(2.4m) x 5ft(1.5m)(25 yrs) -15F(-26C) M Flowers in trusses
of 7-10, 7-lobed, pale yellow, edges deep purplish pink, brownish
blotch; buds strong red. Open growth habit. C.O. Dexter, cross;
Winterthur Museum & Gardens, reg. 1988.

I am not aware of anyone propagating it. The best option would be to get permission to get some cuttings from Winterthur and send them to Kathy Van Veen in Portland, Oregon, who does custom propagating.

Here is a link that might be useful: Van Veen Custom Propagating

Thanks for your reply rhodyman. According to my research Longwood Gardens lists a Hal Bruce also. Last week I spoke with rarefind nursery and they said that they could propagate a Hal Bruce from cuttings also but I would have to wait a couple of years...nothing available now.

Yes, I've been looking for this Dexter since I saw it last June in the Rhododendron Garden at the Coastal Maine Botanical Gardens in Boothbay. They didn't know of a source either. I suspect that this specimen came from Ernie Egan who gave many of his hybrids to the CMBG. I could propogate it if I had a source of cuttings.