Using Google
One of the places I start my search is Google. I would type in Butterick 1234 skirt and see what it finds for me.
For more information on how to do a Google search read:
Ten Tips for Smarter Google Searches
Another reader also suggests trying to do a Google image search.
Using Ebay
If that doesn't work, I try an advanced Ebay search. Google does search Ebay - but is sometimes misses a listing. There is a trick to search for sewing patterns on ebay. You have to enter the pattern company and number and then check the box labeled Search title and description. If the item is on ebay, you should be able to find it that way.
Ebay has saved search feature. You can put your wish list there. You can also contact your favorite vintage sewing pattern seller to see if they have it in their back stock. Make sure you give them all the information so that can search for the correct pattern you want.
Still can't find what you are looking for?
You might trying reading the following articles:
Locating Discontinued Patterns it contains information on how to contact the major sewing pattern companies. The "Big 4" (Butterick, Simplicity, McCall's, and Vogue) are selling their discontinued patterns on their websites.
Try searching Etsy, or Facebook Marketplace or the many Facebook groups that sell sewing patterns.
Happy Hunting!