Around here Ace hardware and the feed stores sell them. Maybe Walmart, Lowe's or Home Depot?

Sets and "yellow sweet onions" don't match. Are you looking for "plants" of Vidalia types per chance. Sets are dried bulblets of hot storage onions. Plants will be difficult to find until late November. Dixondale usually begins shipping in November. Bonnie will be the supplier of most of your local stores, but I don't know when they begin shipping.

I bought onion sets (small bulbs) last year at Lowes. Yellow, white, red, and granex (the Vidalia type). They also had the bunches of small onion plants.