NITRO 27, go to Gunbroker.com, click on Rifles under Firearms in the left hand column, then click on Single Shot in the top window. There are several hundred single shot rifles listed so type .45-70 in the search window on the top right. I would be very surprised if you don't get 75 or more rifles in .45-70 to come up. The Ruger #1 in .45-70 is about the most economical really good single shot in .45-70 at $750 on up; for most of the others you will pay $1,200 on up, and I do mean UP, even for reproductions such as the Browning M1885, and most of them are not as soundly designed as the Ruger. It is based, by the way, on the Farhquarson action arguably one the strongest single shot actions ever designed.
Hope you find one that fills the bill for you.