You'll be lucky to find anybody to screen print full colour at small quantity, and if they do the price per garment will be prohibitive. At least that's the way it is where I am - Ireland.
Full colour on dark garments at small quantity has been the holy grail of garment printing for as long as I can remember. It's basically the toughest nut to crack. Dark transfer, which we don't use anymore is horrible, it's a big, heavy blob on the front of a t-shirt which I always hated doing. If you can get the client to go light transfer - on white (or natural or sometimes Ash grey) you can get a decent result.
For full colour at small quantity we use Versacamm. So your options are to find a shop with a Versacamm or depending on where you are - a decent DTG machine/system. I've never used one that worked out so I stay away from them, aslo they are (expletive deleted

) incredibly expensive.
Hope this helps - good luck!
