There's a place in Germany that has Birkenstocks for a fantastic price, if you already know your size. The site is slow to load, but worth the wait.
www.birkenstock-cn.com
They also have a section of bargains & closeouts. Shipping is free, they are a reputable company, and you'll have your Birkenstocks in 4 weeks. I bought a pair of Boston nubuck clogs for about $38. What a deal!
You should check this place out. I, too, have foot problems and need the support of this type of shoe. Our budget is extremely tight, so this place was a lifesaver for me!