20 amp BLANK FACE gfci on 15 amp circuit

We have an older home where the bathroom is on a 15 amp circuit that isnt on a gfci. We're installing a shower fan and obviously I'm putting in a gfci in the process. The easiest solution for me with the way the wiring is, is to install a blank face gfci as the first thing in the bathroom circuit. The problem is I can't seem to find any 15 amp ones. I've read that it's ok to use 20 amp ones on 15 amp circuits since it's sole purpose is to trip and is not an actual outlet. Is this true?

I can throw in a 15 amp gfci outlet if i absolutely need to since I can find those, but its going to be a lot more work and money.