There is a lot of good stuff on siite. If you're in London they're worth a visit as they're very helpful. Oh - and if you are relatively new to strap-ons I'd start small and work up! Guys sometimes don't realise what they're asking for when they say "I'd love to be taken with a 10" strap-on"
Try making them wear a butt-blug for a while to break them in. are very good for that kind of thing :)