I often have difficulty understanding certain mangas like one piece for example. There are many weird contractions and slangs. I can easily look up slang using slang dictionaries. Looking up contractions, on the other hand, is a different story. There aren't enough resources. There are none on the resources page. I either have to google to find out what it means or to ask on this SE.
Here are some websites I know that list common contractions.
https://www.japanesewithanime.com/2019/07/contractions.html - This is the best one I can find however some contractions are missing like てしまう -> ちまう and でしまう -> じまう for example.
I'd appreciate it if you can find and share other good comprehensive websites/books that focus on this topic. I prefer ones written in Japanese and focuses on the Tokyo dialect.