where can i find a wing nut in size 3/4-10?????

Home depot or Lowe's most likely will not have anything that big. Look in your phone book and see if you have Fastenal near you. They carry fasteners for construction and industry, and their selection is AMAZING. If they don't have it in stock, they can get it. Good luck.

Do you have a Home Depot nearby?

the one near my house has an aisle about 100yards long which is nothing but nuts& bolts, every different size, type, style, whatever of everything.

I'm normally in favour of supporting small business, but nothing compares to the selection of the big box retailers.

You could also look for a 3/4-10 'thread tap'

Lowes should have thread tap sets, and individual ones.

Then you can get a piece of square steel, bend some wings onto it, drill a hole in the center, and use the thread tap to carve the threads into the hole.

your very own real DIY wing nut.

Grainger Industrial Supply carries 3/4-10 wingnuts. You can order online.