I usually
buy the pre-filled carts as it works out cheaper imo - they have filler obviously and you can wash the filler out if the flavor isnt good. I prefer the flat tips but 901 carts fit the 510 atty as well - quite a tight fit (which can be good). I did get a pack of 5 from ecigsupply.com but discovered they were round tips - but the pack of 5 - prefilled with marlbro' high
juice which is nice but a bit sweet - was only $3.50 (the shipping cost more than the carts LOL) so I figured it worked out alright
