Cartomizers are under $3 each in US
You'll still need an atomizer for dripping to test juices etc. And for
vaping juices that don't earn cartos (ones I don't like enough to devote a carto to) or aren't a good thickness for cartos. I would probably just get some Joye 510 attys if it's a Riva 510? I would get them at Liberty-Flights.com if I wanted a bit of a warranty on them (30 days on standard res, 7 days on low res).
The most popular cartomizers for 510 threaded devices are Boge. I use the standard resistance because I move them between Riva 510 and slim 510 (slim ecigs need the standard resistance ones). But many prefer the 2.0 LR ones on fat battery models like Riva 510. The 2.0's will use your charge up faster though. There are also 2.5 ohm ones if you can find them. I get the best life out of Boge cartos with thin light clear juices, no more than 20% VG.
Which Riva do you have? If you have Riva 901 then I don't know where you can get cartos. For Riva 510 I get the Boges at Liberty-Flights.com, LiteCigUSA.net, and elsewhere