What state do you live in? Even if your employee handbook proscribes rules where to buy your shirt, you should still check your state's labor laws. Many states regulate costs of company uniforms or other dictated dress codes. For example, in California your employer must pay for your uniforms or other required on-the-job dress.
If it comes down to trademarks ... If your employee handbook doesn't address the issue and your state laws don't help, then the question is: "Does it violate your company's trademark rights to create unauthorized T-shirts with the company logo?" We're pretty sure you will run into a problem. We think your unauthorized use of the corporate logo is likely to cause confusion (and if done badly, it may dilute the company mark, assuming the mark is famous). And of course, unlike other types of infringement, the company is very likely to learn about the infringement.
And what about using a fake company logo? Our favorite Busted Tees shirt raises a bonus question. Can you make company T-shirts when the company only exists as a figment of a sitcom character's mind. (Or wait, is it a real company ... or companies?) We hope the answer is yes, but either way, we're not giving our shirt back.