Busted gasket that caused a leak ... where can I get this?

Just about ANY hardware store...Home Depot/Lowes/ACE

It's a CONE washer 1/2" for PEX connector

I'd say you need more than the washer. Remove the entire fitting and hit the big box stores for a replacement fitting. Good chance that there's a hairline crack in the fitting you'll only see when under pressure. If they don't have one like it You should be able to find one that will work.

It's a PEX hose fitting...probably 1/4" piping and made by Watts, Seatech (and others). Don't know the size of the brass, threaded end, but it's a standard RV water inlet (ass'y). Try a Lowes, RV supply or a more dedicated plumbing store for a Watts PEX quick connect fitting and do away with the rubber seal. You'll just push on the fitting to the pipe and screw on the threaded end. You should be able to remove the entire water hose inlet for easier access/repair. Kinda looks in a bind, there are elbows that accomplish the same thing as your straight.

just an example

Elbow

Does look like the hose is going to be under a lot of bending stress once it's hooked up. Duster is right..

88-9230 Flair-it 1/2 inch Female pipe thread swivel x 1/2 inch PEX (includes seal)

88-9292 Flair-it SANIPRENE SWIVEL SEAL - 1/2 INCH

You can get that fitting from most Lowes/Home Depot/Ace and etc. You can get a new pex hose and use a Sharkbite coupler(for Pex tubing) to add a new sort piece of hose with the new fitting and reattach and you should b good to go.