Standard sized LED clearance lights in 12 volt are inexpensive enough that it usually isn't worth buying them online unless buying a large batch. Any heavy truck dealer, truck stop or truck parts place will have them. Trailer and RV dealers should have them too. Walmart even has some lights.
A number of the common lighting companies make 24 volt LED clearance lights in the standard sizes. You will likely either have to order those from a local dealer or online. A local bus garage might have some 24 volt LED lights on hand for sale.