As I just had to rebuild mine, and I was on here looking for information, I found most information is old and thought I'd add the latest that I have found.
Petcock (automatic fuel control valve) near the gas cap on the GL1500 is $160 from Honda new and apparently still available, according to the local dealer.
The rebuild kits from K&L are in their catalog but not available on the computer ordering, as many have said months and longer ago.
Show Chrome makes a rebuild kit. Part Number 5-601 for GL1500 Gold Wing. It is a product of Big Bike Parts. http://www.bigbikeparts.com
(I hope this isn't considered advertising.)
These are available on eBay from many sources for roughly $30.
I don't know how the quality compares to other brands. This is the first one I have installed and only a few days ago, but it installed easily and fit well.
Like most, it has a new spring and four new screws. The spring I understand, as you need the best pressure to keep it sealed, but the screws don't really wear out. There are two longer ones for the bottom so maybe they assume people strip the old ones and the longer ones allow them to catch the threads further in.