Generally whatever your pants size is, subtract 21 from it to get your women’s size. For example, I’m a waist size 30 so I would wear between a size 8 and a size 10.
For shirts, sizing up one size has worked for me. I wear a men’s medium so I size up to women’s L. Dress size is a little more difficult but size 8-10 is medium, 12-14 is large, 16-18 is XL to my knowledge.
Shoes are going to follow add two sizes to your current size but be aware that men wear a different width than women. Men’s are generally labeled something like, 10(D) to indicate their width. Women’s will say 10(B).
For places that you can buy stuff; thrift stores might be a good starting place but any retailer will do though. Old Navy and Target have some decent women’s/unisex selections.