I agree, you should stay far away from doubles and really far far away from tripples (unless like me you would use them for display and shows).

Doubles are really intended for decompression diving. Of course they don't have to be used that way. The thing is they are very heavy! You are taking about more than double the weight of a single cylinder.

You would be better off with a single aluminum or steel cylinder. They are available in so many sizes you will surely find something that will suit your needs.

I don't know if you are certified yet, but I would think that would be your best first step. There is a lot of things involved with scuba that are not even thought of as a freediver that the classes will cover. They will surely cover the different scuba cylinders out there if not, make sure you ask.