I second that as well, they do seem hard to find. Something that i have found that sometimes works is sorting the videos by tags - one of which is underwear.

I'll give you an example, here is a helix studios link: https://www.helixstudios.net/videos/65/underwear/?sort=topviews&q=&studio=&tag=65&bonus=

If you click on any of the video's an scroll down a little til you see the list of "themes" (usually called tags by other studios). One of the themes/tags should be underwear and the tag is clickable - that will reveal of the videos involving underwear.

It's not brilliant but it saves you a lot of searching time and usually most studios will have the same kind of system.