600g of sauerkraut before it is drained is usually in a jar labelled as 900g.
If you can't find any fresh assorted small mushrooms, they can usually be found preserved in jars.
I prefer to use whole black peppercorns but you could also grind them if you wish.
Use rock salt or sea salt not table salt, in my opinion table salt should never be used for cooking.
Some sauerkraut is more sour than others, if you find that when you taste the finished chowder it is too sour for your liking you can add some full fat milk and a little bit of sugar to adjust it to your taste.