I can see that you have searched for “air pump” on ebay, this returns pumps to inflate air beds etc.. What you need is a “12v air compressor”. Try this: http://www.ebay.com/sch/Tools-/631/i.html?_nkw=12v+air+compressor&_catref=1&_sop=15&_stpos=&_trksid=p3286.c0.m1538&gbr=1#ptm120798964507
What you basically need is a high volume compressor capable of handling at least up to 50psi (you won’t inflate your tires this much anyway). Otherwise you’ll have to wait forever for the big 4WD tires to be filled (depends on how much they are deflated though). There are high quality products like ARB, Warn, etc.., made specifically for 4WD use, but they do come at a premium price.
I’ve got a good quality Chinese compressor capable of pumping 90 litres per minute. This volume is sufficient for a 4WD tire, it does inflate my car tire pretty quick. Other things to look for in a portable compressor are thermal overload protection and a cleanable air filter.