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https://www.dekudeals.com/ is your best bet IMO. You can set up wishlists there and get notified of price drops. You can also see the price history of games for the last few months, so if something regularly goes on deep discount every 6 weeks you can see the patterns and hold out.
However, if it's first-party Nintendo games you're after, I wouldn't be holding out too much for big sales discounts.
Cheers for the Switch suggestions. I had started looking through what was on sale but the huge amount of cheapo stuff on the store made it a bit tricky to actually pick stuff out.
Do prices change a lot? I wishlisted quite a bit of stuff in the hope that the price would come down but I don't know how much that happens.