Hye Park Lover - Uniqlos are really common in Tokyo so you won't have a problem finding one. But remember there are Uniqlos in London; the clothes are pretty much the same, the only difference is how they arrange the clothes in the mannequins and how they market the clothes.
I suggest you try exploring other shops in Tokyo. The greatest thing about Japan is how there are soo many really small, interesting shops and you don't need to look too hard for them. So just walk around, absorb the atmosphere, and you'll find some cheap and cheerful stores soon enough.
Just go and explore! Don't plan too much in advance!