You need a fill station.
The first one is for use on a scuba tank, the second is for bottles. They also make ones that are for self filling at fields, but you're looking at even more money. (BTW - I don't recommend the online store these are from - I've never shopped there, so I don't know if they're any good!)
Our field actually made a ghetto fill station that was just a nut, pipe and fill nipple thing...dangerous, if you don't know what you're doing, and expensive - but not as bad as the one you can buy.
Edit - just grabbed this link from google...