1) As far as I know. You can only use the dual lightsaber cheat as it stands in the MP game, once you activate cheats via the console command "devmap <mapname>" (eg devmap ffa_yavin)
There is a fanmod that allows it's use without enabling cheats, but I know little more than that - try looking up the u104 mod.
2) Bear in mind that both client and server generally need to have the mod in order for them to work. In the case of your green saber, you might also try cycling through the colours (on the player page, click on the lightsaber) to see if it appears. I'm sure there's a green one there already? Mod files are placed in the gamedata\base directory, with the assetsn.pk3 files, this includes new maps as well.
3) I bought mine in Electronics Boutique, in my local city centre. If you are after mail order, try them, and there are usually a great many other mail order games companies on the net. Buying in a shop - almost any computer games shop should have it due to it's popularity, and if they don't you might try asking them to order it in for you.
And sometimes we like to sleep :> Or even play JK2 - we aren't ALL here ALL the time.