I saw those packages at a few shops that sell JAV DVDs in Akihabara. If you're not shy about it, you can ask the store workers. They should understand some basic English. If not, just use Google translate or something.
I think a workable phrase might be something like "JAV idoru no pantsu ga sagashimasu". That should roughly translate to "I am looking for a JAV idol's panties".
Don't use that phrase in public for the obvious reasons that you don't want to look like a pervert. But within the confines of an adult goods or video store, that should be OK...I think. You won't come off looking too weird...hopefully.
Or you can just look around. Space is at a premium in such shops so the floor area is quite small (and very packed and narrow). It shouldn't take you too long to find. I spotted them just at a fleeting glance in fact.