They are actually not that hard to find. Polymorphing is tricky and in most situations it will only ruin your run if you don't have polycontrol, plus it's effect is short and it can be very effective only if you use a ring of polymorph control and a ring of unchanging that prevents you from changing back in your original form. Scroll of Change Body is kind of polymorph, and it is a lot more reliable and easy to use early on.
SPOILER ALERT. Click here to see text.If you find it then pray to Nefas until she will summon a mistress, then change into her body.