Washed quarz stones are sold in hardware stores for just a few euros a bag of 15 - 20 kg in different size ranges, for example 16-32 mm. Easy to get and cheap, and I did get one of those bags for some "Lichtschacht Dränage" update. But turns out, could use a few more stones than what the bag had. But no need for another full bag at all, even if it's so cheap. Hence the natural alternative is to look for the stones yourself. Which is also quite easy (that's why they are cheap in the first place): just take a walk alone the Rhein river bank, and pick some whiteish opalescent rounded enough stones that match the ballpark of your size range of interest. Did this a few times in different days, and surprise: this was quite a blissful and peaceful experience.

A sort of academic question came to my mind in the middle of all this. Why is it so unlikely to find actually spherical enough stones? They are irregular globs, and I guess the older ones, and/or the softer ones get smoothed out the most. But given thousands of years going through this erotion process, shouldn't the statistics tells us that after enough of it, most stones should end up quite close to roughly spherical? Well, try to find anything close to spherical. It is quite hopeless. Why this is the case escapes my logic.

In any case, if you want to enjoy some peace of mind a bit, I recommend this. Go to a river, bring a back pack or koala bag, and spend some time just picking up some worthless stones. Yes. Spend some precious time not looking into a cell phone or monitor like you are now, but away from technology and walking in the outdoors, looking for worthless stones. Choose your color or shape or whatever insterests you from the stones you grab. You will start getting quite amazed at the endless variations. You will also see quite many little insects here and there moving around whenever you move any stones. It's amazing how much life crawls in those river banks.