I think you should be able to buy online unless your head is unusually large. My head is quite large and they all fit me. They have an adjuster on the underside to snug it if necessary. Be sure to order the soft cloth cap that goes underneath, or it will be scratchy. I bought a couple, but only wore them once or twice because they were hot. Luckily I owned my own business and felt comfortable wearing a head scarf or a hat.

I bought several wigs from Headcovers.com. It’s a really nice site designed for cancer patients. There is an easy guide for determining the size and cap type that’s best for you.
Like @auntieoakley mentioned, buy the soft cap to go underneath to keep it from feeling scratchy and also it helps keep a firm fit. But like others, I really didn’t wear my wigs all that often. I grew to love the ease of no hair and rocked that bad-girl vibe. LOL.