If you live outside the US, and you're sending a SASE to a forum member in the US, you can purchase US stamps online at
I haven't confirmed it, but I imagine they'll ship the stamps to non-US locations.
Postage for a standard first-class letter from the US to Canada is $0.60; since we usually ship seeds with a little padding, it may be slightly more. Two $0.37 stamps (the current standard) will be sufficient.
Can anybody confirm whether the USPS will send stamps to other countries?