Replacement Tires - 20" x 4"

[FocusOnBikes]

After 3000 miles ridden in 14 months of ownership of my 2019 RadMini, I need to replace my worn down tires, especially the rear. I was quite surprised that Rad Power Bikes charges $20 per tire for shipping. That is exactly 1/2 the cost of the tire itself. Does anybody have a local solution (Los Angeles area) to find 20" x 4" tires? Even searching the internet I cannot find very many alternative solutions.