Question
Where in the world can I find weeping birch trees for sale? I would appreciate any help you can give me.

Answer
I don't know where you live but here in Atlanta, Georgia Pike's Nursery regularly carries them.

And my family's retail place "Murphy's Garden Art" also carries one. If you are local or close by call 706-835-7445 for Murphy's Garden Art.

Last try googling "weeping river birch" and see what you can find in your area.
Many nurseries can order plants for you, but typically the big box nurseries can not be bothered to do this...focus on a local specialty nursery with friendly personal service.

Good Luck
Sean J Murphy, LA,ISA, LEED AP
Please check out my blogs and articles on landscape topics like this on my websites.
http://www.seanjmurphy.com
http://amenityarchitects.com