I haven't had to replace the Solera vent covers yet, but I have replaced similar covers in the past.
The covers typically slide into a channel, with tabs at each end of the cover-side channel to lock them in place. I think this attachment mechanism is pretty standard, but it may well vary in some details. Beyond that, the cover lift mechanism has to properly engage a bar attached to the inside of the cover to lift the cover and hold it in place. These lift mechanisms are surprisingly non-standard so you need to be careful to ensure compatibility.
My suggestion would be to remove one of the existing covers and take it with you as you go shopping. I've found replacement covers at RV dealers, RV part suppliers (Camping World and other, local, suppliers) and at trailer supply stores.