Redwood Chair on Turbobricks has a set of these rubbers for sale.
Or if you're doing the rounds of the wreckers, you want the ones from 1989 and a half onward - the ones with little drain holes in the seals.
(pic by Redwood Chair)
These are one piece with no alloy trim pieces inserted into the seal.
The lack of drain holes and the horrible butyl mess in earlier incantations of these seals was prone to trap water and dirt and therefore promote rust. Not these babies, though; add a wee bit of silicone and rope these into place, and problem solved forever.
1989.5 rear seals are fully backwards compatible with all singe rear window 145s, and 245s.