Of if you want blanks, I recommend the Coolcarts at Vapor4Life (which we abbreviate as V4L) or I heard the "Premium" KR808D-1 cartomizers at Litecigusa.com are soft-tip and nice (good customer
reviews on them there) and Litecigusa has really cheap shipping.
Another fun thing I do with KR8 is put a 901 atty on it w/ 901 cart (901 atty screws right onto KR808D1 so it should screw on your adapter too). Soft airy and tasteful and very easy-dragging. I use a standard 901 atty because I just have a regular KR8 battery but if you ever want to try that you might be able to use standard or LR 901 atty with the Riva battery and your adapter? With a 901 atty and carts in your stash, and your adapter, you would effectively have both a Riva 510 and a Riva 901.