There are a ton of threads on this here, but if your looking for a complete budget co2 system here you go:
Easy Peasy solution to Pressurized Co2 on a Budget:
Milwaukee or Azoo Regulator ~$75-$85
Cylinder 5 or 10lb ~$60
Ceramic Diffuser ~$10
Airline/co2 tubing ~$3
Check valve ~$2
That's a grand total of $160. Just fill the cylinder $10-$20 and your good to go. Follow regulator directions.