Found that at Microsoft, too (disclaimer: that was 2 decades ago). Finding good testers at all, no matter how popular your product, is just plain hard. Standard usability testing is somewhat easier. You just put them in a box, ask them to follow some instructions and to explain their thought processes as they fumble through your brilliantly designed product, and sweat blood as they invariably do things you wouldn't dream they'd do in a billion years. Then you pay them. A lot--I find it hard to spend less than $100/tester here in the Seattle area.
