Update 12/11/2006: Due to issues with shipping Weld-On #4, tapplastics.com has decided to no longer sell or ship it. Weld-On #3 works, but the glue time is so fast that I don't recommend it. I made some calls, and found a source in Indiana that will ship to any hobbyist in the continental U.S. He is a small business and you'll have to call to place an order over the phone or via fax. He is not an internet e-tailer:
Quickseal - ask for Max Plumhoss
(574) 272-8695
(574) 272-5004 (fax number for subsequent orders)
Weld-On #4 - 4oz - $4
Weld-On #4 - 1 pint - $7.90
Needle Tip Applicators - $2.65
Note: Weld-On #4 is considered hazardous material in California, and there will be an additional HAZMAT upcharge of $20. Placing an order for a pint of Weld-On #4 and two applicator bottles plus shipping & handling should run about $28.
Weld-On #16 is considered hazardous material nationwide. A 1.5oz tube is $2.25, but there is a $20 additional HAZMAT upcharge. Maybe just look at it as a tube that costs $22.50.