There's a lot of Tshop stops, about 40 or so. Here's a handy reference list I use to track it down, with a description of where it is and the brief look description of the room itself.
The actual Tshop entrance always shows up as the last exit in a room, so it's bolded on this list.
Filligree Street, east of the drum
Sator Square south of the University gates
Plaza of Broken Moons, south of the Hopper Street intersection
The Street of Small Gods east of the Temple of Small Gods
Attic Bee Street west of Short Street
Widdershins Broadway between Bing's Bank and the Lancre Bank
The Street of Cunning Artificers, northeast of Bitwash Street
Upper Broadway north of the Hubwards Gate
The Goose Gate, southeast corner
Five Ways, between Quarry Lane and Quirm Street
Parsnip Place, northwest corner
Treacle Mine Road, south end where it crosses Elm Street
Cockbill Street where it crosses the Lavender Mews
Pseudopolis Yard at its northwest corner
Wharfinger Street just north of the Misbegot Bridge
Eastern edge of the Bazaar
The Road of the Moon southeast of the Ur Gate
Ephebe Road southwest of the Harbour Gate
The Road of the Sun outside a jewellery store
Temple Street outside the Temple of Sessifet
The Hood
Ponder Alley
The Slouching Gardens directly north of the citadel
Eureka Chase
Petulia Avenue
Flatulus Walk, southwest of the Harbour Market
The Genuan docks, outside the harbour master's office
Minaret Street, near its centre
Savannah Road outside the casino
Rue Ephebe outside a toy shop near its south end
Bois village forest crossroads
Royal Market
Middle of Rimwards Road
Cabbage Street northwest of the park
Cabbage Street northeast of the warehouse
Quirm & Hill, northeast of the market
Holy Wood, middle of the main street
Nowhere, west of the crossroads
Sheepridge, southwest corner outside the post office
Dinky, west of the crossroads
Scrogden, at the shadoof
Hillshire, western end
Pekan Ford, at the crossroads
Bleak Prospect, southeast road
Ohulan-Cutash, middle of Long Alley
Mad Stoat, southwest road
Lancre Town, southern edge of the market
Slice, centre of town outside the general store
Escrow town square
Brown Islands, island centre
Thursday's Island