I mainly buy road tubes.
For $5 or so, you can generally find tubes at your local bike store or department store.
Wiggle has pretty good online prices for tubes.
https://www.wiggle.com/lifeline-26-mtb-inner-tube/
About $2 each for MTB tubes (Presta or Schrader). A little less for road tubes, especially if bought in bulk.
Many of the online companies have minimum orders, so things like tubes are easy to add to other orders if you need to stock up on things like chains, tires, or other parts. Hit the minimum, and get free shipping.