Uniforms are a commodity item, sold on price.

Unless you can come up with something unique about them or the company selling them, unless you develop a good USP, it's about price.

How can value be added to these so it's not a price issue?

And don't insult the target client: If I had a towing company and you told me your uniforms would generate more sales for me, I would be insulted and tell you to leave.

You need to develop a USP for the uniforms and the company selling them.

How are your uniforms different, are they more comfortable, do they last longer, does so-and-so use them, are they less expensive.
What??
Do you know anyone selling same or similar items? What are they doing?

Sales letters work better then post cards, you can tell a story, the more you tell-the more you sell.

Are there any other companies using the uniform?

Talk to them. Talk to the person wearing it. Why did they buy them? Talk to the company who makes them. Why do they make them the way they do, etc. See what you can dig up.

And if Joe (down the street) is buying these uniforms for his employees, you could write up a letter, that looks like he wrote it (with his permission) and send it to your target client (this is an endorsed mailing).

Have Joe tell your target client how wonderful your uniforms are, how wonderful your company is, what a great deal he got, etc.

Then you can follow up later.