The shop is found after you have gone totally through Cinderella's Castle (with the mosaics on the walls) to the other side which opens up to Fantasyland. Tink's Treasures is a medium-sized shop on the left as you enter Fantasyland. They have lots of costumes and accessories there...and all things Tinkerbelle and "princess".
Once one enters the shop, "Tinkerbelle" (aka as a series of small optic fiber effects) can be found from time to time as one shops, "zooming" around the store.
She has a regular route, from the "box", through the trees, and across a painted vista on the walls...whatever the route is (my recollection may not be exact, but she "zooms" from the box to 2 or 3 locations in the store before returning to the box and starting her route all over again. I'm not sure, but there may even be muted "Tinky harp" sounds that accompany her short hops around the shop.
This effect is not huge, but can be very magical if you don't tell your children in advance...especially the younger Tink lovers (although the 12 year old I saw last time I was there was absolutely enthralled when she noticed "Tink")
If you truly have a Tink lover in your group, I'd not say a word and then go in to shop for a bit any time of day, but earlier is the least crowded...it may take up to 5 minutes (be patient) before your Tink fan notices her, but when he/she does...it is just so wonderful...it's part of the magic, you see, to be taken by surprise, and not be expecting her!
When my girls were small I was so disappointed when they didn't notice her the first time we went in, but I didn't say a word. When we went back a day or two later, they did notice her! Needless to say they were almost as excited to show her to me as they were that they had seen her...and then there's that thing of, "but darlings...are you sure you saw her?...I don't see her, but if you say you saw her...then I'll just have to take your word for it...what? There she goes again?...but I didn't see anything...Where...?"
When I finally "saw" Tink, that's all my DDs chattered on about for the next hour...and then a recap for daddy...and big brother Joe...and to this day (they're going on 17 now) they love that I played along like that. They say I have to do that for their children some day. I tell them they should do it for themselves, but they say no one can create that truly believable magic but me! Lol...just imagine what I'd be like if I had used my talents for evil?!!! (I do understand/sympathize with the Evil Queen...she's just soooo misunderstood!