Hope you find what you need. As a decal manufacture in the model car hobby for 30 plus years here is some basic info on custom decals.
There is 2 basic methods for very small runs. Less then 50 sets most time are run on a ALPS dry ink printer or one of the newer OKI laser printers that is set up to print white ink. ALPS have not been made in many years and supplies are getting hard to find and very costly. The OKI white laser printers are around $4000 to start. So both of these are limited for home use. Both can give good results if the person using the printer knows how to get the best from it.
Screen printing (silk screen) This method is the best but it requires lots of equipment and a screen needs to be cut for each color printed. And as this all costs something so the minimum run to make it worthwhile is around 100 or more. I spent 2 years working with my screen printer to get perfect non-bleeding white decals. So it can be costly.
I wish there was a good economical method to produce decals in ones and twos., I got into the decal biz to get decals I wanted.....and I still make my own. But custom work is just not viable right now. Laser printers that print white MAY someday get to the point I would get one and start custom work again. Until them you have to find someone, like above, that is willing to do it....for a cost. thx & good luck!
custom decals I did for myself...