They should just be the standard size, 14 x 14".
Definitely get that fixed before any more precip falls. You'll be able to find them at just about any RV dealer, and they should all carry them in stock because it's a common item.
It's best to take the old cover off and take it with you. I think size wise, the three major kinds will fit, but the big difference is in the hinge design. So match up the hinges and you'll be all set.
I believe they should be under $20 each, but I haven't bought one in a long time.
You may want to consider doing all of them at once because if the sun has taken its toll on the one, it may not be long before the other(s) go too.
I believe these on the '07s are new Elixir, but don't quote me on that...
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR