When I moved into my house I had to move 3 pacus (14-18"), a full grown Oscar (13"?) and a RTC (about 20").
I siphoned water from their tank in my condo into three really big coolers that I had in the back of my pickup truck. This worked well because the parking lot is lower than the condo was. Once I got each of the coolers (the big 120 qt ones) about 3/4 full, I caught each of the fish by coaxing them into a 5 gallon bucket, then I'd throw a towel over it and carry it out, dumping the fish and water into the cooler. I just closed the lids on the coolers, since they were only going to be in there for about 40 minutes, and I figured the agitation of the water would give ed them enough air. If you wanted to you could do a battery powered airator, but then you'd either have to drill thorugh the lid, or loose the benefit of having the tops seal on the coolers.
Anyway, once at the new house, the tank and stand were unloaded from the other truck and moved into position. Using trusty 5 gallon buckets, we transferred water from the coolers into the tank until the water level was low enough in the coolers [for the guys] to pick them up and there was enough water in the tank to accept the fish. The guys picked up the coolers and dumped the fish into the tank, and then I just topped off the tank to finish filling it.
Sorry if this was a bit longwinded. If you're buying them from someone else, I'd get as much water in with the fish as you can, and try to use as much of that water as possible because they're alreay used to it (if this is all going into a new setup at your place). I liked using the buckets to catch the fish, and just dumping them out of the cooler because it seemed to be a lot easier on the fish (and us) than trying to net them.
I'm sure there are people with much more experience than I have though, so I'm interested to hear what they suggest.