I am not aware of any exhaustive list of matsuri in Japan (estimates say there are 100,000+ of them across the country in a year). However, there is one trick that I often use and it may help you sometimes to find out more information about very local festivals. The key is to google search it in Japanese language instead of English (or anything else) because these information are often given out on japanese only websites. Firstly decide the type of festival you are interested in. For example, in Spring they have the Hanami festivals (sakura viewing festival), in summer they have Hanabi festivals (firework festival), the lantern festivals, or snow festivals etc. Then choose a local town or not so touristy city where you want to visit, followed by the year. So for example, if I want to learn about the firework festival in Mie prefecture, I would translate "firework Mie 2023" to japanese using google translate and then search using that phrase. You can finally google translate the resulting website if any relevant result pops out.