It's not just the parts vendors going out of business - the parts themselves are obsoleted regularly. There is no attempt to maintain a supply of old parts for future replacements, so when the stock is gone you are out of luck. However, I've sometimes been able to find an out-of-production part by searching the net by part name or number. Often somebody has the part on a shelf somewhere, e.g. a repair shop or a parts distributor. Finding it can be tricky,though.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL