We have been using FlotechLogistic https://m.facebook.com/Flotechlogistic/ for our ISC West and Asis booths and our show room.
Find a retail space in your own community and then ask who designed it. Here is a site that names the top retail architects. The top one (Gensler) i am familiar with has offices in DC. Big buck firm, but deservedly so. Note that they do small projects as well as the large ones.
I think it will be tough for you to find an all in one designer and builder. we went to a few experienced architectural firms who have worked with big clients in the past, some didnt want to waste their time with such a small project but others did. We narrowed it down to one and they designed us a concept that a typical contractor could build. I would recommend that you find a few good architecural firms to have them design you a concept as they will have designers on staff that can do that sort of thing. Our showroom is pretty small and we are not a billion dollar company but I really wanted to make my showroom look sweet. We ended up spending $4500 on just the design concept of a very small area but the design is sweet and ended up being much better than anything we could have thought up. I then looked for an experienced builder who is proficient in designing modern things, I asked the architect for some recommendations and we went with a guy they referred. The building costs will end up being $13,000 so as you can see we spent alot on the design compared to the actual costs, but again, was worth it as I really wanted to make an impact.