Depends on which era you are wanting. I find searching with various terms helps, but I usually have to search with a really broad term to get the obscure stuff :) I find 1940s sewing patterns/1930s sewing patterns good search terms.
Pants are rarer before the 50's. You might try looking at the repro pattern companies for those ones.
Now, if you are OK with the 70's, there are lots of pants patterns from then. You might be able to take 80's patterns and slightly alter the pattern to look more 50's/40's like.
Decades of Style have two pants patterns:
Vintage Pattern Lending Library have:
Eva Dress has:
1951 Frontier Pants (unsure if men's or women's pattern!)
1936 Snow Suit (with pants)
1946 Lounging Ensemble (with trousers)
Folkwear (if you can ignore their terrible 80s photos!) have:
Rosie the Riveter Pants (1940s)
Simplicity have two reproduction 1940s pants patterns, 3688 and 4044 (eBay link).
Searching on ebay, I usually use search terms of the decade, "pattern", and in your case, "trousers", "pants" or "pyjama/pajama". I leave out "vintage" or "sewing" as it will sometimes exclude items that don't have these words in the title.
When searching on Etsy, I usually put in the decade I'm looking for, "pattern" or "sewing pattern" and occasionally a garment type (eg. "1940 sewing pattern dress"). When searching on Etsy I always make sure I select "all items" as you never know how some sellers list their items. Here are some links to search results for terms I entered on Etsy (I thought that would be easier than putting in individual listings as I wasn't sure what size you wanted):
1940 sewing pattern pants - several are for pyjama pants but looking at the patterns themselves they look workable as pants. I wasn't sure what size you were looking
To potentially get different results, you could try substituting "pants" for "trousers" or "pyjamas/pajamas" (spelling dependent on country of seller!)
I hope that list is of some use to you, I've had fun putting it together for you! Let me know if any links don't work :)