Craigslist is the best. You can get about 15,000 knex for around 100$.
Ebay or a car boot sale
Ebay, many people sell hoards of it, for very cheap.shop.ebay.com/i.html
Garage sales are very good sources of K'nex. You can get them in large quantities and for very cheap and sometimes for free.
Either yard sales or eBay. I've also heard that Goodwill sells used Knex.
Nope. I've been to the Goodwill store a few times, no toys or anything.
All Goodwills are different.
! There's oodles of toys and that stuff!
Yard sales have cheaper knex, but they are held less often and around here, such a thing is extremely rare. A better bet is Ebay.
Yardsales, since their customer base is so small.