Ich sage
Google "T-shirt design mockup PSDs" for Photoshop templates of T-shirt mockups, where you can overlay your own designs.
MOZ
It is an ineffectual template, since it has fabric folds where the design should go...This makes the design look totally fake, and certainly would not help you sell the design OR the shirts...
Why don't you simply ask your designer, instead of trying to sneak around behind their back? There are no exact matches with a Google Image search, as most t-shirt templates have the chest area smooth, so as to best display the artwork without screaming "FAKE!" So if you want that specific fake-looking template, you should just ask your designer. It may be their own photo, in which case you can be sued for copyright infringement if you use it without their permission.