a good belt?

I think my belts are slipping.... after installing my new alt.... i can see belt dust on the new alt.... Where can I get good belts.... I know gator back is a reliable maker... But everything I have found is that our cars are special order only... btw my SC is a 90.

And I might as well change both to keep them even on wear n tear?

gates or dayco, they make the best for 8 rib belts for our cars. napa for sure has some and most advanced auto stores do aswell.