Anything that lists as a KR808D-1 cartomizer will fit.
I know VaporKings sells them 5 for $8.25 (only slightly better price really, but better flavor selection)... there are others too, just that VaporKings is the only one I've used so far.
Really, to get a better budget going, you should look into re-filling them yourself. Then you can pick any flavor/strength that you want.
I had some trouble re-filling my 808s and eventually gave up and moved over to a 901 model (808
batteries fit that by the way, so if you ever go that route, it's just buying attys and carts... just FYI there). But a lot of people here seem happy with cleaning and re-filling the 808 carts just fine, so I'm more than likely messing something up.
I can't post links yet, but if you search YouTube for DoctorVapor he has a whole series of videos on refilling cartos that I thought was helpful.