Source One is Burchard of Worms' Decretum, Book XIX. The Decretum was a collection of canon laws compiled in the early half of the 11th Century. Book XIX, or The Corrector, was a penitential: basically a guidebook for confessors. Here's a sin, have u done it, here's your penance.
One of the questions for women was, essentially, "Did you make a dildo, strap it to yourself and fuck someone with it?". The original text is in Latin, and there's a few translations floating around of what it said. Here's one, which I spent the past three days looking for, because I wanted a direct source:
Have you done what women are wont to do: to make a certain device in the form of a male member to the measure of your will, and to tie it to your own or another woman's genitals with some ties, and commit fornication with other women, or others with the same instrument, or with another with you?
- Translated from Latin, taken from "Die Bussordnungen der abendländischen Kirche nebst einer rechtsgeschichtichen Einleitung", F. W. H Wasserschleben
Pretty cut and dry re: the use of strap-ons. And dildos, because the next question is "and did you use this device on yourself?"
The second source is from the trial of Katherina Hetzeldorfer, specifically Female Sodomy: The Trial of Katherina Hetzeldorfer (1477) by Helmut Puff, which has an analysis of the trial as well as a translation of the trial texts.
Katherina is the first recorded woman to be executed for homosexuality. There's a lot to be said about her and the way she performed gender but what I'm interested in today is the strap. So, from the court text itself:
...She made an instrument with a red piece of leather, at the front filled with cotton, and a wooden stick stuck into it, and made a hole through the wooden stick, put a string through, and tied it round; and therewith she had her roguery with the two women...
And there we go! Two sources about people in medieval times using strap-ons, one from around 1020 and one from 1477.