u have 2 choices to get cheap air.
1. buy a pre-set bottle as Pikachu suggests at the low to mid 100's
2. buy a second hand angel air, max flo, armageddon etc for a bit more (200 ish)
whilst the second one will cost you more in the short term it will be more flexible in the long run. you also need to consider how you are going to fill the bottle unless you are lucky enough to have a site that provides air (ooohhhhh heaven
)