I don't think there is a replacement cup for the Stanley 1913 Classic Vacuum Bottle, I haven't found one, as an alternative you can buy the Stanley 1913 Adventure Nesting Shot Glass Set 59 ml so that you have more cups.

1900 ml Weight: 1470 g Category: Drinking bottle Material container: Stainless steel (18/8) BPA-free Insulating capacity: 24 hot, 32 cold h Dimensions Height (mm): 37 cm Closure material: Polypropylene (PP) Bottles & Mugs Properties: Double-walled Type: Insulated bottle Closure type: Screw cap with integrated seal Thermal function: yes Drinking bottle With integrated mug