If you think you will need loads consider making them in long lengths, you can get spindle cutters for it pretty cheap. https://www.whitehill-tools.com/profile-cutters/domino-dowel/132126C/I often make oversized loose tennons and mortice them out with the domino.
If you dont have a moulder then its just a roundover bit in the router.
Getting a variety of 8mm & 10mm beech dominos, but Trend don't seem to do 12mm... they do 4mm, 5mm, 6mm, 8mm & 10mm "only". I do have a 12mm cutter for the DF700 but only use it on big stuff, like garden benches, which I'm not making at the moment.
The Festool ones are hard to beat on a price quality basis. But if want a lot and your time doesn’t count then you can easily make up your own. There is no need to round off the ends, just 45 degree trim the sharp edges off. You can also make some wider ones for the wide wiggle setting. The trend ones don’t look as good as the festool ones.
I got some from eBay which were a lot cheaper. Sadly, they were a little bit too wide and so on the narrow setting, they would often split the workpiece. On the wide setting, they were fine. But as Jerome says above, probably better to get the proper ones.