At any GOODWILL store in the USA.
Why would you want "used" "BOYS" undies? Are you a man in his 30's and loves the smell of used boys undies? Or are you just looking to save a few bucks and force your kids to wear some other kids underware that was prolly pooped and peed in and depending on the age of the boy he could have even let some things go in his sleep and its stained for ever!! Buy new undies unless your a pervert.