I don't know what you're going to put in the gun so I don't know what kind of gun you're looking for. Have you 'asked' the Pest Control companies what they use? Are you asking about something like a Garden Sprayer? More info would be useful.
In the meantime, check out this link: Chemical Sprayers - (Look towards the bottom of the page for 'Spray/Foam Gun'.)
Again, more info would be useful. (WHat are you using it for and what are you putting in the gun? ANd can you even describe the gun with some details?) For now, my best to ya and hope this helps.