Translation:I always buy clothes from this store.
Just a fun tidbit. ようふく（洋服）Literally means western clothing because at the time when this word was created everyone wore kimonos/kimono like articles of clothing so any clothes that weren't kimonos were from the western world. That is why it literally means western clothes.
However the meaning now a days has changed from 'western clothes' to just being clothes or outfits. Now 洋服 is a generalization of all clothes. It can be pants, shirts, dresses, or whatever.
ふく（服）is also a generalization of all clothing as well. You can use either one.
(Don't know how reliable this is but I've heard that Japanese people use 洋服 more often but is typically related more towards women's clothing than men's clothing which is why 洋服 can be loosely translated as 'outfits'. Again don't know how true that is.)
In English, we might say "at this store," but never "on this store." If we are talking about online shopping, we might say "on this site," though. While the action is the same, when we use "from," we are generally thinking about the store as the owner of the object we are buying it from, like a person. When we say "at" or "in," we are thinking of it as a place. The "de" in Japanese actually sounds more like "at" in English, but I don't know enough Japanese to say whether there is another way to express that.
In Japanese, you can say にて to mean "at", but that's essentially just the formal version of で.
I also wanted to mention though, that "on this site" in Japanese would also use で but here, the particle is being used for its "by means of" function, rather than "location indication".