I get mine from K&L but any good local shop should have a selection from the big houses. If you want smaller, family run wineries, you’d have to go to Portugal.
Retail, either local or on the web, for old and current release.
Auction is a good source for older bottles since:
a) prices tend not to go up much with age (unless you’re chasing legends), and
b) Port is robust enough to survive less than ideal storage.
I’ve noticed that as well. But I bet the price has gone up a lot for the 1985 compared to its release price, as will the 2016s 30+ years down the road. But for me, if you want birth year wines for your kids, your only way to assure it is to get them on release.