If you have money, definitely go with a professional designer. At first if you don't have too many projects requiring a designer, just hire one on a contract basis. If you start feeling the need for a full-time designer then it's going to be beneficial to hire one.
Good looking free icon sets are available, but you should shop around for a decent icon set which you can reuse for most of your projects.
Finally, if you don't have access to a designer, just keep the look very clean and simple, because chances are that you can't do a good looking design (since you're not an artist).