x2 on the dickies. They are almost the standard when it comes to uniform shirts. They come in 100% cotton and 65/35 cotton/poly which are more durable and maintenance free. If it's for a uniform that will be worn everyday, I'd go with the poly cotton. Cotton shows wear and wrinkles easily. I've used one of their long sleeve premium industrial work shirts for years now as a hiking/working around the house shirt, and it still looks new.

You can find them in any walmart and target it a wide range of sizes, and quite a few discount clothing stores sell them. Cost is about $20, maybe a couple dollars more for X-Tall and 2X-4X sizes.