You will not find a reputable source willing to ship you a single chick - there are several reasons for this. The first issue is that shipping in and of itself is such that a single chick is unlikely to do well. IF the single chick made it through shipping and arrived alive, you then have the issue of brooding a single chick which is problematic and ill-advised. Furthering the issue is that, if you successfully brood a single bird the integration process is harder when you are introducing a single new bird into an established group of birds.
The best solution for your situation is going to be to order the minimum, brood them together and then sell the "extra" birds off. By having them beyond the brooder stage, you can sell them at a price that will pay for the whole order - plus you have birds that have grown out and you can be sure of the gender (yet another issue of starting with just one chick is if you are the unlucky winner of the "cockerel lottery" you are 100% loss vs. 1 out of three/four/five).
