Depending on where you are, every major department store has got a wig section. Also, amazon, but that can be a hit or miss.
you didn't specify what you wanted to use the wig for, if a photoshoot to hide your identity or due to hair problem. Wigs don't look very natural in photoshoots, I tried myself in the past and it made my head look huge and it just didn't look very natural. It might look better when its a short-haired/ bobbed wig.
Else for longer/ denser looking hair in the photos, I can recommend clip-in extensions, they will work out far cheaper if you plan to use them just for one occasion such as photos taken.