I have no idea where you can buy them, but they wouldn't have to be stubben. just make sure they're the same color as your saddle. good luck! maybe look on Ebay? people are always selling random stuff on there.
The tack store I use has them and you can buy them separately. They are a bit expensive though. You can also get them with the velcro, so you can move them or take them off. Have you tried phoning some of your local tack stores or calling someone who fits saddles in your area?
I bought my saddle from Advanced Saddle Fit in New Hampshire...mine has the velcro/removable thigh blocks and she had a whole boat load of knee/thigh blocks, would reccomend giving her a call to see if she had any. Even if there were only ones with velcro... prob wouldnt be to hard to removed velcro to have put permanently onto your saddle!
They have a website w/all the contact info
Advancedsaddlefit.com I believe!
the "I'm In My 20's and Hope to Be a Good Rider Someday" clique