What weights to use for fermenting sauerkraut?

Penn State Extension recommends:

Cover with a plate weighted down with jars filled with water or cover with a large food grade plastic bag filled with salt water (6 tablespoons salt per gallon of water.)

I can't see why not, pie weights should be fine. The ceramic weights used in large kraut crocks are unglazed ceramic as are the pie weights. I would place the weights in a plastic zip bag though, to make sure they don't drift downward, maybe even filling the bag with the brine as well. Afterward, you could always bake the pie weights to sterilize them between uses too.

Get a set of ceramic weights that fit a 1 liter Fido jar.

https://www.etsy.com/listing/182083359/fermentation-weights-for-1-liter-fido-or

(disclaimer: this is my own product)

A quick tip: whichever way you weight your kraut, add a piece of outer cabbage leaf (make sure all your veg is organic, not sprayed) under the weights to hold all the bits of kraut under the brine level. It gives you a good wallop of the right bacteria too!

I use these glass weights, there are quite a few varieties available but the basic idea behind them is the same: they almost completely fill the opening of a Mason jar or other jar which has the same diameter opening (many do). The stone weights in a dedicated fermentation crock do the same job, minimizing the kraut's contact with the air, but the glass weight is one piece instead of two so it's a little harder to get them in and out of the jar than it is with a stone weight like the one Leslie linked to. There are versions for either standard-mouth or wide-mouth jars.

I use the book Making Sauerkraut and pickled vegetables at home. In the book it states to use a water and acid-proof stone such as granite. Limestone or marble will dissolve in carbonic acid. It also states that a board that is either birch or beech are traditionally used for this purpose. Do not use pine or fir, because their strong odors can be transferred to the vegetables. If you use a mason jar or a big gallon jar you will need to cut the weight in half to get it inside the jar. Only fill the jar to about 75% capacity with the vegetables or cabbage, add your stones or boards and if using boards a bag with the salt water solution on top. Making sure that the brine solution is at least an inch above all. Finally after sealing the top I use a rubber grommet in the lid and attach a fermentation lock (the same one I use for fermenting wine) this in turns allows no air to make contact with the fermenting goods, and thus prevents the mold or kahm yeast from forming.

Although weights are highly recommended, I have successfully made cabbage saurkraut without weights. When not using weights you have to make sure you pack down the cabbage nicely, the juices drawn from the cabbage should almost fill the jar. leaving very little air in the jar. Everyday you will have to pack down the cabbage as some of the cabbage bit would be floating and may cause mold formation. This works for me for cabbage and beets.