I am looking for the peep sight inserts for the 99A (mailbox style) peep sight on my Winchester 75?? Anyone help??

I'm not sure what you mean by mailbox style. Brownells has replacement inserts for several makes of target front sights.

http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=12372&title=GLOBE+SIGHT+INSERTS

There's a number of rear apertures by Williams and Merit at Brownells too.

Bye
Jack