Fast Tech I think has best deals but if you are traveling soon you may not have the time to wait on there shipping. I use them for carto's, clearomizers, standard ego tanks etc. It's much easier to refill just about anything when using those. The standard bottles vary in length and hole size. The needle tips just make everything a lot easier.
Another suggestion is to buy a few 10 ml or so ones. I fill them up with my 30-50ml bottles. It's even a good DIY starter experience as I mix varations of vanilla, caramel, tobacco, and coffee flavors into a 10 ml needle tip bottle. If you know what you like its a very easy DIY experience for mixing flavors.
I've yet to take the next step of "actually" DIYing but this gives me ability to try things out without wasting a whole lot of juice. I've recently got a cherry flavor that I'm too found of but if I mix in some vanilla, caramel, tobacco juice it's not too bad. Better than pitching a bottle of juice that you don't like.
Anyway my 2 cents on how those bottles have helped me.