anybody can typically buy from vwr.com, fishersci.com or sigma-aldrich.com. But these are companies that typically cater more to research and pilot scale type programs. Meaning you are going to be paying a significant premium for the chemical. That said you'll be assured of the purity.
You should be able to get boric acid from numerous places...Home Depot Walmart etc. Should be in the pest control aisles.
You can also get it from amazon.com
http://www.amazon.com/Orthoboric-Con.../dp/B00K2DTJAC
Likely can get it on amazon.ca as well.
Note boric acid for this purpose may also be called orthoboric acid
From your friendly neighbourhood boron chemist.