To explain, T-BOT is talking about a different sort of transfer entirely. You will need to order plastisol transfers from another company, usually with a minimum of 12+. They ARE good quality, but will not work for single prints or jobs like that.
As far as opaque transfers go, the most recommended are the Transfer Technologies transfers and the Blue Grid transfers - so you might give those a try. Personally, I haven't found any opaque transfer to be good enough quality to use myself, but other people think differently.
The primary other method available for small runs on dark shirts owuld be vinyl. You would need to get a vinyl cutter to do this, but the quality is good and you can print just 1 at a time if you want to. However, as with all methods, this one has some limitations as well. Specifically, you will only be able to do a few colors (usually only 1-3 or so), and you have to manually weed out the excess in the design (which is easy or very hard depending on the design).
Finally, direct-to-garment printing is the other option available for this sort of printing. Unfortunately, these machines tend to be very expensive, and many of them have trouble printing the color white very well.