The letter sizes were used on bias ply tires. Since this system was discontinued in the late 1970s it is possible you have the original tires.
Vintage Tire Size Conversion Chart - Vintage Car Connection - Old tire sizes
I would suggest a modern trailer tire of similar dimensions such as the ST225/75R15 in load range C.
The H78-15 had an overall diameter of 28.4" and was used on 6" wide rims. The ST225/75R15 Goodyear Marathon has an overall diamater of 28.3" and also specifies 6" wide rims.
Some forum members recommend the use of light truck (LT) tires instead of trailer tires, though this is controversial.
You can check prices at tirerack.com but you should end up paying around $100 tire. I have found it more practical to buy tires at a local tire shop. The choice is yours.
If you have one-piece rims you can probably reuse them depending on the condition they are in.
You may have two piece rims which most shops will not service. If that is the case you may have to replace the rims.