http://cgi.ebay.com/NEW-CORNELIUS-CO...QQcmdZViewItem

Add a needle valve(5-20$mx) and you have a new regulator for about 25-45$.
Fire and ice, beer supply places sell tanks often for cheap.

Diffussers?
See 2$ DIY on the venturi CO2 reactor here.
Or the 1.70$ air stone from Sweetwater at Aquaticeco.com

So for about 20$ (+ ship cost) for a reg, 10$ for the valve, 2$ for brass adapters, 2$ for the diffuser + the going rate locally for a used 5-10-20lb tank(thjese run 45$ for the 10lb filled) you can go gas tank CO2.

Cost 79$, plus some shipping, maybe 15$
Still, under 100$

Regards,
Tom Barr