I have a bunch of RX packs from CheapBatteryPacks.com, and like them a lot. They have a great selection of cells of various capacities and as mentioned, they will weld together a pack in a custom shape if you want. Prices are quite reasonable.
On my Spektrum brand RX's, I've been moving planes over to 6v Nimh packs of 2600 or 2700mah capacity, which are standard AA cells and work great. I do have some Spektrum RXs on 4-cell packs and they work fine as well, I'm just going with the extra cell for insurance on some of my nicer planes.
For a charger, you CAN use your wall wart that came with the radio system. It WILL charge a 5-cell pack, it just takes longer, and it should eventually charge a high capacity pack, but it will take days to do so.
I don't recommend you use the wall wart. And if you do, be sure to use a voltmeter to check the pack. I've seen more than one guy use the same "charge for 10-12hrs with wall wart" routine on a 2000+ pack and then crash because the charger wasn't getting the battery fully charged.
You will be much much better off getting some other charger. I use the Ace Digipulse 6-output charger here:
http://www.acehobby.com/ace/ACE2521.htm
Because I don't have to think about how many cells in which airplane, and it will do even large packs with enough time. It says it only does packs up to 2000mah, but it does fully charge my 2600 and 2700 packs just fine. And having 6 outputs means I can keep several planes and TX's charged at once.
There are lots of other options for chargers though. If you get one of the peak detection fast chargers, those are nice to have, but be aware that you really want to do a long slow charge the first time or two on a high capacity Nimh pack or you won't get even close to the rated charge out of it. And new Nimh packs often "false peak", fooling the charger into thinking the back is fully charged when it isn't. So use your voltmeter.
Btw, I also picked up an 8-cell 2700 for my JR 9303 TX, now I can fly all day easily, or get a couple of flights in over 2-4 days with out charging.