The easiest way to do this where you can personalise them is an OKI White toner printer and T.ONE paper from The Magic Touch. This will give you amazing results on a variety of substrates and colours. If you want to do a strong white on a dark garment you would need WOW paper instead, but as a user of both papers, whenever possible I use T.ONE.
You could use TTC paper on a standard OKI laser printer but the finish will not be as good and not something you would want to sell.
T.ONE paper costs £1.40 per A4 sheet (I think) and takes 5-8 seconds to press and 10 seconds to seal
You must have a very good heat press (Adkins or equivalent) to use this paper.
If you are serious about this sort of gift ware range then pop down to The Magic Touch and spend a few grand

(I will see you there collecting my commission LOL!!!) Alternatively and depending on if you like to take a risk, there are a few second hand OKI White for sale on
www.printerexchange.com
Lastly, other people will recommend other papers that might be just as good as T.ONE. I have no idea as I have never used them, I use TMT with their Space Control which saves a considerable amount of money on toner etc.
USING: Whatever it takes to get the job done...