Lactobacillus reuteri naturally occurs in breast milk and is sold as an individual probiotic, so you wouldnt need to buy the breast milk unless you're after the individual immune factors that it contains. Plus not all humans carry L reuteri. A company called BioGaia developed it and licenses it out to different companies around the world. The patented name is Protectis.
I make my own kind of kefir by adding a crushed tablet or emptying a capsule of probiotics to a jar half filled with oat milk. I just leave it in a warm dark place to ferment. At least you know what strain you are getting. I've done this with L. reuteri, L acidophilus and L. plantarum etc. You can pretty much take your pick, even S. boulardii works fine.
It really saves you money, plus you get a lot more bacteria in one cup than you'd ever get by taking a few capsules.