Most of the places I know of that sell this kind of tea now have some kind of HK or SE Asia connection (with the exception of Muzha Tieguanyin from Taiwan). I think the HK shops are generally the best bet. I like Lau Yu Fat's, though their top grade is not cheap (3rd grade is also decent, and more affordable). Both Best Tea House and Aroma have branches in Canada, so also worth considering just in terms of convenience (BTH, especially, is more concentrated towards the middle of the roasting spectrum, but I think most of their teas are well balanced).
Some folks have also had good success withhttp://www.just4tea.com/Tie_Guan_Yin.php
See also this thread:viewtopic.php?p=80640
You can also find stuff from Yao Yang (Geow Yong) on Chinese auction sites. I think it's a bit overfired, but if you like a really strong roast, you could look at that.