Buy a bottle of olive oil and a bottle of vinegar (a fancy vinegar if you want, apple cider or balsamic, but not malt) then buy a packet of Italian spices (typically a blend of oregano, thyme, etc...)
Buy some salad greens, either a head of lettuce, or a box of spring mix or similar blend of greens.
To make salad dressing mix two parts oil to one part vinegar, put in a pinch of Italian spices, a pinch of salt, whisk.
Then pour over your greens, add some feta cheese olives and baby tomatoes, capers if you want.
But your typical large-format grocery store will have one. It's hard to talk chains and be comprehensive because American supermarket chains are not national. But whatever your market's equivalent of Wegmans/Publix/Kroger/HEB/Meijer/Albertsons/Hy-Vee should have one.
Start checking out your local grocery stores. In addition to selling, well, groceries, most of them will have a deli section and a salad bar. Salad bars in grocery stores typically get charged by the pound, so you can pick and choose exactly what you're getting. Most grocery stores in the US will also have already-cooked rotisserie chickens, sides available by weight, and other precooked options. Most large grocery stores in the US basically have a fully stocked cafeteria in them.