A Superformance dealer can get them (hint, hint!) However, they are not in stock in the US and are not cheap, not that any good Hartswell type latch is.
Fran Kress has some repros that are quite nice (had one in my hand this weekend but he caught me so no good deal there....) Just find an Embraer 190 aircraft on the tarmac and help yourself. Jetblue has a bunch.... jetblue embraer 190 - Google Search
Hello Eric
Re: clip latches or repro Hartwell latches.
I bought a set off a chap called Mark, they were perfect replicas apart from the word "PRESS" was engraved instead of embossed as the original latch tongue was cast and the repro's are machined. He has sold out but he is thinking about making some more. They were £500 a set of 4.
Alternatively Gelscoe in England may have some but they may have bought them from Mark.
Here is a link to a filler cap on ebay from Mark and you will be able contact him that way. It is what I do. GT40 FUEL CAPS ORIGINAL CAST ALLOY CORRECT SM450 SERIAL | eBay