My boss just asked me whether I knew any good places to advertise for good sysadmins for a big-scale data-centre build/spec in the UK - and I didn't have a good answer. Do you? If so, what? hiddennetwork and joelonsoftware spring to mind; anything else?
I would refer you to this classic article: Finding Great Developers
Anyways, my thought is that advertising isn't enough, why would a good clueful administrator leave their current job and join you guys? That's part of what you need to consider and include in any message you try to send to these people ($5000 per year training budget, 4 weeks holidays, whatever you think will float their boat). If you create an attractive position you should be able to hit up your network and hopefully one of them will either go "sod it, I'm switching jobs" or "hey this would be perfect for Bob, I'll forward it to him".
I'd start with http://www.inqjobs.co.uk as lots of British sysadmins are fond of its parent, theinquirer.net