I use these rigid toploaders. Crystal clear and won't allow the yearbook to bend/crease. They come in different sizes, thickness and prices, so just shop around you'll find one to meet your needs....
https://www.ebay.com/itm/16560013200...Bk9SR7rimIGIYQ
You can also buy them in multiples...
https://www.ebay.com/itm/18503134620...Bk9SR7rimIGIYQ
https://www.ebay.com/itm/16454897004...Bk9SR5De_YGIYQ