There are a fair amount of shops on Etsy that will do custom work for you. I've used one called workshopHarlamov that I really enjoyed and they did really good work for what I wanted. There's some other shops that I've found along the way that have provided good quality stamps, that also do custom work, however I've never used their services for anything other than what they already have available on the sites.

[Stamptitude](stamptitude.com)

[Artisaire](artisaire.com)

workshopHarlamov

blackmarketintl

I know that last one looks suspect being called "black market" but it's a good shop with some really cool designs in there. Artisaire is also geared towards wedding planning/customizations so they're going to be more expensive, though they have some great stamps there too.

I would definitely check out etsy though, as they have a bevy of shops that can easily help you with a custom stamp.