Tractor supply has them also.
One of the reasons if chose that particular relay is, it's readily available everywhere in the USA, and inexpensive.
I prefer not to call it a lawn mower relay(it is used in hundreds of thousands of riding mowers), it is simply a relay. It's used in tractor trailers, snow plows, and all kinds of other vehicles and industrial equipment. But, you can find it in the mower section of most stores.
The most important reason it was chosen is, it's internal design. By design, it will not fail as the original AC40 relay can and does.