First, check with booth vendors in your area. It is important to have local access to your vendor so you can easily go to the showroom to see the products, get ideas, and also be able to easily get support, extra parts, and supplies. Most sales reps will also bring a booth to your office and set it up so you can see it firsthand in your own environment. Many times, booth sales reps are also great resources for ideas for designing your booth. They know what works and how to make your booth effective. Use their experience and advice -- it's typically free! Check references of the vendors you speak with. Talk to their customers and see if they are happy with both their booth and the service from the vendor. Or, go to a local trade show and ask some of the show vendors about their booths and whom they work with. You are about to make a potentially large investment, and a little product research is very valuable.
Also, many vendors will store your booth in their warehouse or showroom while it's not in use, for no additional charge. Some will also make sure it's clean and in good shape, and ship it for you when you need it. Make sure you know what they charge for this service (if anything -- some vendors provide it free) because those fees can add up.
Usually, the booth vendor can either create your graphics from images you supply, or they may offer services to create the images for you. Here, cost is usually the key difference. If you have an internal graphics staff, you'll save money, but make sure they are in good communication with the vendor graphics staff to ensure that the appropriate sizes and formats are supplied.