The Source (aka formerly Radio Shack) Some of them stock silver solder.
Acklands (may require an account) In the catalog with the welding supplies.
Air Liquide (or any other welding supply outfit)
Some of the places that stock the BernzoMatic line of propane torches and accessories will have it. Look for the bags with three or four rods for various purposes. Check CTire or Home Depot.
Air conditioning guys use it to join household A/C lines. Yellow pages, under Air Conditioning Supplies.
Or get a Brownells catalog and see what they have (lots!)
If you have oxy-acet, the welding places are probably the best. They may require that you buy a certain minimum qty. If the rods are not flux coated, you will need flux too.
Cheers
Trev