This is a difficult question to answer. My business is mobile which means I don’t have to worry about these type of headaches ;-). I agree with Ariadne that the answer here is “it depends” on what you are looking for. First, you need to make sure you are at the right location for your business (accessibility, traffic, visibility, safety, ect.). Also looking for the right size within your budget is very important. You don’t want to go over and at the same time you need to know how much space you’ll need for the type of services you will be providing (you don’t want to be paying for space you will not be using for example).
A real estate agency can help you find some options you might not have thought of yourself. You have nothing to lose by using one of them and it might actually save you some time and money at the end. You are the one who will make the final decision, so no matter what they come up with, it will not go through unless you agree.