There's a few questions within this question. The Joye 510 and 306 have the same connector and so you can use a 306 on
mods designed for the 510. The 901 has a completely different different connector and
thread. Adapters are available to allow you to use 901 attys with 510
batteries.
As for burning atomizers at 6v, be aware that at 6v they are drawing nearly 3x more power than they were designed for. Assuming a resistance of 2.4 ohms for a standard 510 atty (it's about what I've measured on a few examples) at 6v it's drawing about 15 watts. At 3.7v they draw around 5.7 watts. At 5v they draw around 10.4 watts. So the power increases exponentially with voltage. The more power, the hotter they get, and the greater the chance of burning it out prematurely. You can check these calculations here -
Ohms Law Calculator
So, what to do about it? Well you can carry on and accept that you are going to get through a lot of attys. I think the technique is to take very short drags to minimise the burn time. Or you can use high voltage attys which have a higher resistance. Or you can drop the voltage and use say, a 18650 battery instead of the two RCR123's. If the standard attys do not do it for you at 3.7v, try a low resistance atty. These typically have a resistance of 1.5 ohms and @ 3.7v perform about the same as a normal atomizer at 5v. They tend to be cheaper to make up for the fact that they normally have no warranty.
Afraid I can't help with the cheapest supplier for attys in the US. I've used Eastmall (they stock the high voltage and low resistance types) and their prices are very good on bulk attys.
