Remember originally that -1 was 1 BC, meaning 1 year before the birth of christ. The negative numbers are measuring the distance away from 0.
Edit: in the positive direction, the 1 was 1 AD, meaning the first year of our lord. Just like when talking about the reign of kings/queens, the first year of their reign is 1 and the 14th year that they reigned is 14. I believe the timekeeping for Ages in LOTR may also be similar.
Only other place I can think of off-hand is the local library for rentals. Half Price Books and thrift shops will have some for sale, but I’m not sure how much lower they may be from Amazon’s rental fee.