Just watch the PSI ratings on the eBay ones.  
The ones Gardiner sell are only rated at about 50 or 60 psi I believe.  Anything rated higher will work, but will flatten the battery quicker.   
I use a 5 lpm / 60 psi one on my trolley and get just about a days use from a 10ah battery.  If you want a longer service life from the pump and fancy a bit of DIY electrical work, you could rewire your backpack with a 40amp relay.  I'd only recommend this if your warrantee has expired though.