Ashley, I don't have anyone in particular to recommend, but all this task requires is access to the chimney, the price of a good cap, and a half hour's time to install it — perhaps less. But if you don't already know the dimensions of the flue, that could involve another trip, and so more expense.
Also, the type of cap may depend on why you need it. (Typical reasons are: water intrusion, racoon intrusion, bat intrusion, etc. The types of caps needed will vary according to the particulars.