Do you want them for everyday riding? I bought some for showjumping as my mare knocked her knee and it swelled up quite alot but never any damage. I bought the caldene ones they were fine whilst warming up but wouldnt stay up for jumping. I got them from equestrian clearance.
I've tried a few different knee boots but am yet to find some that don't move or fall down - have to admit I've only tried the cheaper brands though. I've heard good things about the Jeffries knee boots but they're very pricey.
If its for roadwork then I'd go for leather which won't rip through and stay in place if you do go down. The jeffries ones are great and I got a pair on ebay at low price. I've tried other leather ones but they didn't work which would be no good if your horse did go down.