It depends on the exhibitions.
So exhibition organizers only you let you use their stands/stalls with limited options for changes (less common), some refer you to a list of 'approved' stand installers/designers (fairly common), and others let you do whatever you want so long as you follow a few basic safety guidelines (and stay within your stand limits). For the last option, there is a wide variety of service providers.
I am in touch with an international stand creator - they create very unique stands, but they aren't cheap. If you have a high enough budget and are in the US or Europe, I could connect you.