They are very fast compared to electrical, why not make your own for a
fraction of the cost?
I use a second hand rotary (for size) air conditioner pump with home
made bracket. Bare in mind with this setup you cannot grease the
bearing, however I fit a "Uni Filter" to the intake and keep it damp
with a little auto trans oil. I know of one bloke that doesn't use any
lube, just plums the intake into his air box and recon's he's been on
the same compressor for years (little use maybe?).
At $50 a pop at the local wreckers (last time I bought)it is a cheap
alternative.
Some people run the air into a tank, (an ex fire extinguisher is good
for this as they come in a variety of sizes) with a pop off valve, or a
truck brake tank is OK as well, but I find it works just find straight
from the compressor (tyres, air beds ect).
If you can't fit one to you engine you can make a portable one using a
starter motor with a pulley attached. Just make up a frame and mount the
starter motor next to the compressor and run a short belt, a set of
jumper cables and off you go. Note: be shaw to us a starter motor that
has a front bearing, not one with a bush and shroud as you wont be able
to fit a pulley to it unless you cut of the shroud, and then the starter
wont have enough support at the front to take the load of the pulley
(although this can be overcome). Desirably,fit one that just has the
motor body a shaft sticking out, like one of the old pommie one's, or
one of a small truck, a search around your local wreckers should get you
one.
Another system I have seen on several Ser 11 and 111 4 cyl landy's is
where the compressor is mounted to the chassis via a bracket and when
you want to use it you just wind a belt onto it and start the engine,
good if you really want one and just can't fit it to your engine.
I have used both the engine mounted one and the portable, you will be
surprised as to how compact you can get the portable setup.
I remember comming off Stockton beach one time after a clean up day with
roughly 50 other 4x4's using anything from expensive electric pumps
through to el cheapo's and hand pumps.
I pulley my portable out and had finished all four tyres before some had
completed one, managed to draw a crowd that day....bet a lot of em
headed home and straight to the workshop.
Of course these portables can be setup with a tank as well.
Cheers
S

ive got a setup mounted on my fj40 landcruiser using a sanden 508 a/c
compressor ive got a 26l tank built into my rollcage ive pumped up the tyres
on 4-5 vehicles one after the other and its still going strong  i can run
air tools from it aswell so greaty for changing tyres on a trip with a
rattle gun

well worth the setup costs
$50 compressor
$20 arb pressure switch
$50 hosing and connectors
$50 tank
$  brackets or if your 4x4 came with the optioin of aitcon find the bracket
from one and use that

at 1500rpm the compressor actually cycles on and off while pumping up tyres