Personally I just do a web search for "<something> battlemap".What ended up happening in my case is that I found a creator (Morvold Press) that was consistently making maps for the adventure I was running (Storm King's Thunder), so I joined their Patreon. It worked out great.
😂 Right?! I often ask myself the same question.
edit: Doh! I just learned this is a meme! 😅