Greg has some good links on his website:
http://thamnophis-alba.m.webs.com
Including links to several of the European guys. Sjoerd from CrazyReptiles posts on here fairly frequently, as do Udo, and Gijs and Sabine. There's a few garter/natrix people around in the Midlands, there's a meeting on Wednesday where I hope to catch up with some of them. I'm not sure who is likely to be breeding what this year. I went to the IHS meet at Doncaster in September and there wasn't a single garter on a table, really annoying that such great snakes are so difficult to find.
I know that crazyreptiles are coming to Donny this year, so there will be some garters this year.

I would suggest checking the RFUK classifieds forum, it's worth keeping an eye on, but not many garters come up for sale on there. The forum does give you hints every now and then about who may be breeding interesting species this year.