I vape almost exclusively with KR808-D1's and have for a year now. I've purchaed both batteries and different cartos, etc., from about 10 different vendors, including ones suggested by others here.
I now purchase all of my KR-808D1 blank cartos (regular size) from LiteCigUSA.net FOR use with my short or XL KR808D1 "slim" line batteries. I prefer the hard caps but they also have soft caps. Good price. Easy to refill. Refillable many times. Cleanable. These are the right ohms (as explained below) for regular (or even XL) batteries.
BUT . . . . . these cartos do NOT work on my KR808-D1 USB passthroughs that I also purchase from LiteCigUSA.net.
I also own several vGO's - KR808-D1s from Kanger. Love them - last much longer. These are much higher mAhs - 1000 and 1300. With these I use the KR VGO XL Empty Caartos from Vapors Heaven. These work great on the vGO's, hold about 2x the amount of liquid, and work with BOTH the vGO and the specific USB passthroughs I have. Also refillable multiple times, easy to refill and can be cleaned.
Note - I don't use the USB passthru connected to my computer. It's connected to a USB adapter with A, W, V to match the PassThru -- then connected to an extension cord to a wall outlet. Works for me and saves my computer from potential damage.