I went to Etrailer and Raney's. I also picked up a set of 7" round generic led lights, amber and red from a school bus supplier. I'd be willing to sell those for a bit less than I got them for because I changed my mind on the style.
Depending on the style of your bus, your front turn signal is also a parking light, requiring a 2 function light, while the ambers on the rear do not.
Depending on your budget, the 7 inch rounds go for $30 to $50, sometimes $60 or $70 for specific lights, like the front amber one.