I buy them from knife making supply companies when I place orders for my knife shop. That being said, I believe most Hobby Lobby's, Lowes, and Home Depot's will carry the BSI epoxies.
Alternatively I have in the past also used some common two-part JB Weld with excellent results as well. However, since jumping into knife making I just use the quality knife making epoxies over buying something specific for arrows. I like the BSI and G-Flex because they give a long shelf life, long working time, are easy to clean up run outs with acetone, dry CLEAR (instead of yellow/brown/grey like a lot of other epoxies), and have relatively high strength as compared to most of the quick setting varieties.