True "Safety" color are always a 50/50 blend, because the fluorescence cannot be achieve with 100% cotton. But we do have options.
Port brand 100% cotton tee. It's a good brand (made by Hanes, actually) and, if I'm understanding you correctly, they'll be going thru them quickly. So you'll want to keep it affordable.
Here are a few bright colors. They are BRIGHT COLORS, but not NEON or SAFETY. Shown below, and in the header above, are Aqua, Red, Royal, yellow and Orange. The lime green has been discontinued.
These will be your most affordable option. 100 pieces with the 4-color logo printed is 9.40 each. 200 pieces 7.95 each. The larger sizes come with an additional few dollars, depending upon the size.
Bayside is an American-made brand that caters to the American Worker. We have an option available here. Not true safety colors but these are called BRIGHT colors. Orange and Yellow are available. A bit more $ for an American-made product, but the colors might be closer to what you're looking for. 100 pieces $12.25 each. 200 pieces 10.50 each. The larger sizes come with an additional few dollars, depending upon the size.