Reddit mentions of Lenox Tuscany Classics Double Old Fashion Glass Set

If it was a glass for me, I'd probably get a pair of the Glencairn mixer glasses (not the snifters) that are recommended above, but unless your girlfriend is a real whiskey snob she may not recognize that you've gotten her an "official" tasting glass.

If you are looking for something unique or something that she can enjoy both looking at and using, I'd recommend either of these. They catch the eye from across the room and are sure to be a welcome replacement to her plastic cup:

UNIQUE!!! Rox and Roll 4 Piece Old Fashioned Glass Set
https://www.amazon.com/dp/B00GBFDJME/ref=cm_sw_r_cp_api_Gl1lzb10XCQMS

PERSONALIZED!!! Custom Engraved Old Fashioned Double Wall Rocks Whiskey Glasses - 7 oz - Personalized for Free
https://www.amazon.com/dp/B01N29R0KY/ref=cm_sw_r_cp_api_Ts1lzb33S8P61

CLASSY!!! Lenox Tuscany Classics Double Old Fashion Glass Set https://www.amazon.com/dp/B0095VPOKQ/ref=cm_sw_r_cp_api_KC1lzbVSN3P2A

I also personally like the Iceberg glasses that are on Amazon, but that's sort of an acquired taste, I like that the top is square and the bottom is complex, but you'd have to know that it matches her style.

Good luck!