How to buy difficult-to-find electronic parts?
Go to the manufacturer's website, find their list of franchised distributors and talk to them. However don't be surprised if you have to buy a whole reel/tube/tray and wait a few weeks, but you might be lucky and find someone with stock. You may be able to get small qtys as samples from the manufacturer or distributors. 
However there are many parts that you just can't buy in small quantities.