Your goal is to find just the right graphic design studio, so here's a plan. Do a search for graphic design studios, and even localizing it for your area, there will the tons of responses. And, in today's internet world, it's not necessary for the firm to the local. That only broadens the scope of what's available to you and increases the difficulty of making a decision.
So, how do you choose? Look first at what you need and it will be much easier.
Being clear about what you want and need, and able to clearly communicate that, will make it much easier to see which of the graphic design studios is a fit. Are you looking to focus your marketing efforts on print or the web. Do you already have a logo or do you need one? What about CD or DVD production? Make a clear list of what you want.
Who are your customers? Perspective graphic design studios will need to know all about your target audience, so be ready to answer the "who", "what", "where", "why", and "how" questions about them. Define your potential and current clients in terms of their demographics such as age range, income range, ethnicity, lifestyle, professional focus (if applicable), education, home ownership, and mobility.
What are the reasons they buy your product or services. What pains or problems do they experience and what solutions does your product or service offer? What are the inherent benefits to the customer of buying your product or service?
How does your target audience use your product or service? What were your past marketing efforts? Why did your past marketing efforts work, and why not? If you don't know, that's okay. Bring along your past marketing materials and ask your candidates what they think. This is another way to assess whether or not you want to work with a particular studio.
Knowing your product or service, your market and what it is you want is just half the equation. The other half is what you are looking for. Here are some simple questions. You can rank the candidates on each of these on a 1 to 5 scale where 1 is "not at all" and 5 is "Yes, absolutely".
Do they have access to all the services I need? You might want to make a list of the services and then rate the graphic design studio on each one.
Does the style of their work meet my taste criteria?
Do they have experience with my product or service?
Do I think I'll enjoy working with them? (This may not seem as important until you're in the middle of a project. Then it will be crucial.)
Will they give me references?
Are the references happy with the work they received?
Okay, now you are armed and ready to choose from among the bounty of graphic design studios. Remember the ones that come up first in your search know how to optimize their web sites and if you are working on a web site, this may be important to you.