The Kirisame Magic Shop is the only shop that can be found in the Forest of Magic in Gensokyo. It is a house built in a Western style that just screams "A witch lives here!" at anyone who looks at it. The sign out front is handwritten, which gives it a bit of extra visual spice.
Though the word "shop" is in the name, the shop does not really have items for sale. Instead, it is just filled with stuff Marisa has collected. While there is (barely) enough space for someone to live, people other than Marisa would probably feel suffocated by the claustrophobic environment...
Marisa hates staying still, so standing at the counter of the shop is fairly difficult for her. She usually offers her services instead of selling stuff so that she can keep moving.
The Forest of Magic can be confusing to navigate. If you try to head for the Kirisame Magic Shop, you might end up at Alice's residence instead. Perhaps Marisa and Alice are neighbors.
If you ever have a request for Marisa, it would be much smarter to head for Hakurei Shrine instead of the Kirisame Magic Shop. Marisa is often away from the shop, and you are likely to find her at Hakurei Shrine instead. As well, the shop is in a dangerous place for normal humans. If you want to head into the Forest of Magic, you should make sure that you are properly prepared.