There are opaque transfers that you can use on dark fabrics, but a lot of people don't really like them because they're thicker than the transfers for light fabrics and feel a little "plastic-y". Personally, I think if you get the right transfers they only feel a little thicker than apparel vinyl, and a lot of people use that.
The other thing with opaque inkjet transfers is that some people have problems with them fading and cracking. I'm still conducting tests to see if that will happen with the opqaues that I have.
As for plastisol transfers, yes, you will need someone with a screenprinting setup, but most people would advise you to have those done by someone else (I think). There are several companies out there that do these and for someone just starting out, I'd suggest using one of them if you decide to go the plastisol transfer route.
The advantage to inkjet (or laser) transfers is that you can do just one if you wanted to. With plastisol, you'll need to buy several of the same design (tho you don't need to press them all right away).
