Harbor Freight sold an air booster years ago for about $100, it used shop air to drive pressures up to 500psi.
Used one for a few years on aircraft, easier to use than a nitrogen setup in that you got roughly 5psi out of each trigger pull, as opposed to filling then removing air to desired PSI.
I'm sure the more expensive nitrogen fillers have a valve you can preset for a certain PSi, but the one I was using didn't.
Anyway, I've been trying to find one for quite a while now, they disappeared from HF's shelves, and I can't find one that uses shop air to compress to the 500-1,000PSI range, much less one that costs less than $400.
Follow this link to see how they work, it's at the bottom of the page (this unit uses scuba tanks, but same basic principle):
http://www.advanceddivermagazine.com/articles/babybooster/babybooster.html


If anyone finds one like I'm describing, let me know!