Selection
The most important thing to start with is finding a retailer with a large selection of MMA t shirt choices. You don't want to end up with a t shirt you're not crazy about, simply because the store you were shopping with didn't give you many options. The more choices you have, the more likely you'll be able to find something you really love and that will fit within your budget as well.
Shop Online
Shopping for an MMA t shirt online is likely to be your best bet. Online stores have lower overhead costs than traditional stores do, and as a result they can usually offer you a lower price. You'll also find that shopping from home is simply easier, as you don't have to drive to store after store or worry about when stores in your area are open. Of course you're likely to have a much larger selection online as well.
Consider Future Needs
When you're looking for an MMA t shirt, remember that you're likely to want other MMA gear or UFC gear in the future. Look for a store that has a wide range of not just t shirts but other types of clothing and fighting gear. The best case scenario is finding a company to supply your MMA goods that you can grow to trust. When you have future purchases to make, you'll then know immediately who the best company to shop with is for you.
Value
You don't want to overpay for your MMA t shirt but you also don't want a poor quality shirt. You can probably find a really cheap option, but when it shows up and is thin and comes a part after its first washing, you'll find that it's not a very good value. You want to start by focusing on finding high quality t shirts. Once you've found the quality you want, you can then compare costs to see who offers the most competitive prices on quality merchandise.