I do not know where to get 50 pre-filled cartomizers. I chain vape and I am sure I go through more than 3 / day. I really cannot tell with my technique (described below). When I first started vaping, I bought pre-filled carts and empty carts. I know that the carts are the same, but it appears that the empty carts can be refilled many more times and are more productive than compared to the pre-filled carts. I believe that the atomizer in the pre-filled carts gets gummed up or maybe something precipitates out of the juice onto the atomizer, rendering it less productive. Basically, I have been testing this (I am a scientist), and so far it appears that the empty carts can be re-filled considerable more times than the pre-filled carts.
As for refilling the carts, most people would not recommend this, but I have not had a problem (yes I use an automatic). I just drip 6-8 drops into the battery end, vape on it for a while, and then add 6-8 drops again. I never get the burnt taste. I can easily do this over 20 times (I have not counted) before the production of the cart decreases dramatically. I do wipe of the battery end of the cart before I attach it to the battery to remove excess liquid because I have heard that getting liquid into the battery can break the switch or something (automatic battery only)
For the above filling technique, go to vapornine.com website and they have a video showing this.