There are a number of places (some of which I obviously can't mention in the forum).
If you already have OS 9.0.x, you could apply the appropriate updater to get to 9.1
macintoshgarden.org might be a good place to start.
If you're looking to buy the CD (and your internal CDROM drive is working), make sure you get the retail version of 9.1 CD which looks like this and is denoted by the number 691-2746-A (this is the US-English version). The international English version looks the same but is marked as Z691-2746-A.
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