LIberty-Flight.com and click the 20% off on front page
LiteCigUSA.net (get a pack of standard resistance Boge 510 cartos too) (under $3 1st class shipping)
And you have to get the charger cable for the 510. Slim 510 cannot use Riva's charger cable (can fry a slim battery) and slim 510 cannot use LR attys or LR cartos so if your Riva is coming with LR attys, slim 510 cannot use them - can fry it.
And look at the 510 mega xl manual battery (280mah vs 180mah for standard 510 battery). The mega xl is long though - 128mm w/ a Boge carto, longer w/ atty+cart.
Buy MANUAL 510 battery if you want the battery to last. Manual 510s are totally sealed on the threaded end.