Check out Canis Gear.
They sell in bulk and have shelter package of mixed collars and leashes. I believe you can request free collars for shelter. I do not know the process or details on that.
We are using martingales on our dogs, and a size medium pack of 10, is roughly $32.00.
They also have rescue dog shelter packs. Which is 21 sets of martingales or collars with leashes.
They also have a link for $6 Collars, for individual purchase. Collars are well made.
We also utilize Campbell Pet.
Again, they sell in bulk but can also buy individually.Collars are also well made. It looks like they are updating their website and selling more things now.
Hope this helps!