Don’t worry about the websites – the process has been the same for 25 years. There are lots of suppliers, they all use the same inks and the same adhesives. Provide them with an editable PDF, AI, EPS, some will accept CDR.
This guys are in Santa Ana (orange county), California. We’ve used them for over a decade now. Small shop and family run business. Most design firms and schools around this area have used them. Regular INTs, reversed, different substrates, etc.
70’s (new) website but great service. You can ask for Jose or Luis.
This company is based in New York. Image Transfers specializes in making dry transfer decals for industrial designers. But because vector images can be ganged up on a single transfer sheet, these custom dry transfers can be quite affordable and cost efficient. They make museum-quality rub down transfers for product or packaging prototypes and can deliver in 24 hours. See lots of their recent projects on their blog.