Asklemmy

If you are working with a specific company to produce a personalized / custom product, they will generally have at least one person on staff that can do design, or at least deal with any file conversion and technical stuff like that. They should be able to quote you a price.

If you are working with an artist first, they may have a company they're familiar with to have the item produced, and if they do, should be able to handle most of it themselves. If they don't, or you would like it produced somewhere specific, just let the artist know - most places will have all the information the artist will need on their website, or the artist can just get in touch with them directly.

In general, if you're going to an artist or design shop they should be able to give you some kind of quote with a very rough idea of what you want - especially if they do this professionally, talking to people who don't have an art background is basically half their job. If you want to be specific it's much more useful to send something like a mood board or a collage of similar things than a bunch of text. It's also generally a good idea to be open to their ideas, as they often have experience that may be helpful and will see some problems much quicker than you might.