I don't know anywhere that you can buy them, but sweet potato slips are SUPER easy to start yourself and way less expensive (if cost is a factor for you). We start our own each year from organic potatoes that we buy from the grocery. We use the "potato 1/2 in, 1/2 out of water method" too, however since we need a lot of slips, we use a large (8x15) stainless steel pan. I can usually fit 10 or more potatoes in the pan. I lay the potatoes on their side and put the pan in a warm sunny spot in my diningroom. When the slips get about 6 inches long, I pull them off at the base and put them in jars of water to grow out the
roots (take off any leaves that would be under water otherwise). You don't have grow out the roots, but in my
experience, the slips are less likely to fail if the
root system is already started before planting.