For contemporary out-of-print sewing patterns I love searching the Pattern Review website as they often have good quality images of discontinued patterns that I can then download to use in the StashHub app 
(including description text, line drawings and envelope back sizing/chart info.)
For vintage patterns I often can find images on Vintage Sewing Patterns | Fandom (they have a good database of patterns from year 2000 and earlier).
I do this too. A lot of my patterns are oop. It’s really hard when the numbers have been reused.
@ClaireSews thank you - Pattern Review and Vintage Fandom were the two most useful resources while I was doing the cataloguing exercise.
@AmyinSC yeah I reckon in hindsight the pattern companies wished that they’d used maybe a 6-digit format to number patterns 
Another resource for out of print pattern pictures. Commercial Pattern Archive - Home
I just tried with one for which the photo I had was dreadful and found the photo and a picture of the pattern pieces.
Hope this helps.
@JD3205 ooohhh, nice find on that archive link 
. Thank you for sharing it
