I am a volt user and it is very easy to fill their cartomizers, not to mention cheaper. You can refill them for 20-30cents. To me the easiest method is the condom fill. Just take the condoms off, fill one a little over halfway and slowly push one end of the cartomizer in the condom. I fill from the mouthpiece side. Just take the end cap off with a paper clip or something and push that end in the condom then sit on the table for a minute battery side down. Once your done, put the end cap on , give a quick blow through, wipe off the battery side, and your done.
Vapedudes sells 100% vg, their Flavors are good and they have a promo for a free bottle of juice and a 40% off coupon. Just remember, higher vg will clog your carto faster and give it about 30 minutes to soak before using.
You can use vapor4life cartos, but they are not as good. The have a different type of atomizer and they burn easily. The volt cartos last a long time and don't burn easily. I have been using the same 5 for a month now and they are still working well. I fill them every day