To be honest now a days it is cheaper to source the work out than make it yourself. I used the below company when I started to do Personalised hoodies
Personalised Hoodies custom personalised printed Hoodies UK before I moved on to a niche market where the money was to be made
Since many of these big companies stock their own products they can produce it cheaper than a small printer.
If are going to do volume I would personally suggest contacting a Chinese company and import it yourself since that would work out cheaper for you in the long run. Yes their is a wait but if you plan it right then these guys are quite efficient.
Good luck