Take an old one with you for sizeing. Lowes, HD, Meards just about any home improvement store will have or can get what you want. I suggest you get the ones with the dampers on them. I replaced all mine when we had a 2001 Adventutrer 35U. Was able to control how much heat came out of which vent.
I had also put fiberglaass screening round them to keep a lot of dog hair from going down inside the vent.
Tom & Bonnie
2008 Winnebago Aspect 29H
A Class C Not a B+
2010 Ford Flex
We wire brushed them and painted them a hammered bronze as I recall. It matched the old color pretty closely.
Since we put down laminate flooring DW is not happy with the color so I will have to repaint them in a few days.
Clay WA5NMR - Ex Snowbird - 1 year, Ex Full timer for 11 years - 2004 Winnebago Sightseer 35N Workhorse chassis. Honda Accord toad.