It looks like I can get dark red, dark blue, teal, purple, safety orange, safety yellow, grey, black and white individually for a reasonable price in adult sizes small through XL.
I think that should provide a nice range of options for most folks. I will still need to print and apply the heat transfers myself, but that is doable.
Sizes larger than XL will cost more and I haven’t found a good source with variety of colors for those yet, but there likely won’t be too many of those needed so the average cost should still be good.