I got a slide topper from PPL Motorhomes (online)
RV Awnings, Slide Out Covers, Screen Rooms, Sunvisors - PPL Motor Homes
They have been the least expensive for most of my aftermarket purchases.
I installed it myself and had no problems. Came out very nice. Just follow the instructions. As mentioned before, it would have been easier to have a helper.
Some come with covers which I did not get. Wish I would have because the birds keep going under and making nests.
Lots of choices size, color, fabric etc.