Also look on CL and the for sale items here on HBT for kegging items, such as kegs, CO2 tanks, regulators, etc.
I've ordered all my kegging and dispensing supplies (QDs, tubing, taps, shanks, etc.) from Ritebrew.com. Their pricing is whole lot nicer than NB or Midwest (same company as NB). I bought a CO2 tank with double regulator and a bunch of kegs off Craigslist.
Most places will only swap CO2 tanks, so you'd never get to keep your brand new shiny one. Call around to see what's what. You shouldn't pay more than $1.50 per pound of CO2 for a 10# or 20# fill or swap. But a 5# swap/fill may run you $10-15.