I have never grown Sweet Potatoes - as with other root crops they’re ridiculously cheap to buy in Wisconsin.
For your next growing season, I’d look to purchase them as soon as regular potatoes or Sweet Potatoes are for sale in the very early Spring in your growing zone.
Looking on-line they look pretty expensive for slips! What do you give for the potatoes themself at the store? If they are organic and/or have not been sprayed with a sprouting inhibitor (they should be able to tell you) you could probably start slips yourself. ;)
I did find you some recipes though, so this thread isn’t entirely useless, LOL!
https://insanelygoodrecipes.com/japanese-sweet-potato-recipes/