Like you've said you can snap your own pictures or you can buy a package from people like GoMedia that contain all kinds of realistic apparel that you can use for virtual samples.
Here is the GoMedia "Ultimate Apparel Template Collection"
Ultimate Apparel Template Collection
Here is a tutorial on how to make a mock-up using the templates above:
How to Create Photorealistic T-Shirt Mockups – Graphic design tutorials, freebies, & advice by working artists and designers. | GoMediaZine
It contains all of their apparel pieces and they're fully editable (color changes, etc,.) and they're also high resolution. Of course there is also the option of buying their packages separately (t-shirts, hoodies, etc,.) as well.
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A second option would be to download them from a website that offers templates for free.
Outlaw Design Blog Free Blank Vector Clothing
The only drawback with some of these files is that they're not very realistic so some of the "wow" effect is lost when you do a virtual sample on one of them as opposed to doing one with photo-realistic material.
In the end it's up to you how you want to present your samples. I've personally done both of these methods and I don't think there is a wrong choice. Do I think the GoMedia templates are worth it? Yes.
If both of these 2 options don't suit your tastes then maybe a quick search for "realistic apparel templates" or something like that through google might yield better results.
what are the best options for making sample images for a clothing line website? 
