I use Dicor sealant for most of my repairs including the roof. There are some other brands just as good and you can get the sealant at most RV supply houses. Don't forget Camper World.
If you can't get the parts from the vendors you might try an RV salvage yard that is close to you. If your close to Florida I know of one. Just let me know that you want the location. Nerver been there but I thought I might camp close and see what they have.
Welcome to the forum.
98 Endeavor DP, ISB275
RX300, Falcon II hitch, BB Vantage Select
VMSpc, 2002 Grey Ford Powerstroke
TST 507 w/ 10 Sensors