I need 16 21700 battery cells, but at a decent price for a project I'm working on. Any good website recommendations for buying them/accompanying hardware for a custom battery pack? What about brand? Are Samsung's batteries any good?

If anybody has good information on a decent 3s BMS board that can go with this pack I'm putting together, that would be great!

 

There's several good suppliers out there. Google is your friend.
I've bought 21700 and 18650 cells from Liionwholesale.com . Top quality, genuine cells

Stay away from flee-bay. It's full of counterfeit lithium cells

 

I bought two Li-PO batteries from my local hobby store and they work perfectly.
My friend bought 10 cheap batteries from ebay and he gave me one.
Many of his do not work, the remainder work poorly and the one he gave to me does not work.
Buy cheap then get cheap.