The slips you do yourself are FAR better quality than what you can buy anywhere. And they are almost free when you use last year's harvest. This is from my garden notes (I do this every year with reliably good results in z7B)
Sowing. Around Mar 1st, find a good mother potato - one that may already be sprouting. Then, take a small slice from the stringy root end and make sure flesh is uniform (no streaks, pale spots). Then plant tuber in pot with light soil and water. Keep in a warm, moist, light place. Fertilize lightly after slips begin to form [4-6 weeks]. Harvest slips when ~6” w/4-6 leaves by pinching them from the mother potato w/some skin, removing some of the bottom leaves and placing them in water where they will grow roots. After ~ a week and sufficient roots, xplant to deep flats.
Growing. Grow on in flats 3-4 weeks until mid May. About 10 days before planting begin hardening off. Plant when soil temp is >65 deg. Loosen the soil for better yields, add compost for drainage, and side dress w/ 1 tsp S of P. Plant into damp soil – set slips vertically for large roots and horizontally 2-3” deep for average roots and higher yields. Place 3-5 leaf nodes underground and keep plants damp. Secure tomato cage over plant and encourage vines off the ground. Little growth occurs 1st month as roots form. 2nd month vines form. When vines stop growing, roots are forming for the 3rd and 4th month. Count 4 weeks after the first flowers appear to determine harvest time. Harvest before soil temp goes below 60 deg or potatoes become hard and do not soften w/cooking. Dig roots w/hands only to not damage skin.
Protecting. Bunnies love the leaves. Planting root crops in bins/pots seems to work ONLY if sparsely planted and sufficient P included along with other nutrients. Sowing must be 50% less dense than when sown in the ground.
Storing. Do not wash – some dirt on skin is good, just remove excess dirt by lightly brushing. Cure sweet potatoes by holding them for about 10 days at 80-85°F and high relative humidity (85-90 percent). If the warmest temp is lower, say between 65-75°F, the curing period should last 2-3 weeks. To maintain the required high humidity (85-90 percent relative humidity), stack storage crates or boxes and cover them with paper or heavy cloth. Packing in perforated plastic bags will also keep humidity high, yet the perforations will allow excess moisture to escape. During curing process the skins thicken, the cuts heal over and the starches turn to sugars. Curing is complete when rubbing 2 potatoes together does not damage skin. Once the sweet potatoes are cured, move them to a dark location where a temperature of about 55-60°F can be maintained during storage. Sweet potatoes are subject to chilling injury, so keep them out of the refrigerator. Outdoor pits are not recommended for storage because the dampness encourages decay. Good results can be obtained by wrapping cured sweet potatoes in newspaper and storing them in a cool closet. Set several in plastic bags not touching each other to prevent drying out