I would go with the suggestion that someone made about waiting a couple weeks before getting a new employee uniforms. This is what they did at the furniture company I worked for.
Gives ya some time to get a feel for whether they're gonna stick around or not.
I also liked the suggestion about getting them some regular T-shirts that are a close match in color to your normal uniforms to kinda hold them over until you do get them uniforms. And this way they don't go to a customers house wearing any old thing that could make you look bad.
One other suggestion would be to make them front the money for the uniforms then reimburse them after you see that they're gonna stick around (say 3 or 4 months). Just a thought.
Good luck either way! Take care.