There are many places you can find vintage patterns both online or in person.
1. Yard Sales/ Estate Sales/Thrift Stores
Despite how popular vintage sewing patterns have become, there are still people who don't know what they have so good deals can be found at these places! But you will have to do some digging. Mostly I have found that this category yields mostly 70s and 80s patterns with the occasional 60s jem mixed in. Always keep your eye out though! I found this great 40s jacket interfacing at a thrift store. Be on the look out for sewing notions too!
2. Antique Malls/Vintage Stores
These places have a much higher concentration of vintage pretties, but you usually have to pay for it. Like with the first category, I've found that the 40s and 50s patterns are extremely scarce still with 60s onward being easy to find but investigate your area for yourself. Different parts of the country will have different things. If you area is lacking, try to convince your family to let you sneak off to an antique mall on your next vacation. Don't forget to look for basics. A great pencil skirt pattern is great whether it's from the 40s, 80s or modern!
I personally don't like paying more than $10 for a pattern. Maybe $15 if I really like it. So I generally search for patterns and sort by lowest price first. You do have to sort through lots of children's and men's patterns first using this method but there are gems hidden too! The other thing I do is set a price limit to stop at. Then I don't even see any of those pricey patterns! Sometimes if I'm looking for something super specific, I'll break this pattern and sort by relevancy but I do try to keep my favorites full of cheap vintage patterns.
I don't particularly like ebay. The auction style makes me feel pressured to buy on the spot and I much prefer to leisurely ponder patterns and then purchase when I have the money. But there are deals to be found on ebay. You can also find lots on ebay.
5. Other Online Vendors
Some other online vendors have stand alone websites where you can find vintage patterns for sale. I don't use these too often because they tend to have higher prices and less selection than etsy. However, if you can catch a sale, then you can get some good deals. These patterns as well as last week's 40s pattern haul came from Vintage Martini closeout pattern sale which is still ongoing. (They're stopping selling patterns.)
Next up, how to determine your size in vintage patterns and what to do if you find one you love that's not your size.