You can find some on Amazon, good prices as well. Etsy has some, pricy I found. Of course Bad Dragon sells all types of knotted dildoes. Try with a golf ball first, if she get that in and out she can start with a med. A hockey ball, it's plastic
Have fun, post some pics or vids of her progression. As far as a first time with a dog, recommend being tied down, there's no turning back at that point and she'll know if she ever wants to do it again.
I'm a femboy/cd so there's only one place it's going. I was tied and I was knotted my very first time. It hurt and I don't think anyone is ever truly ready for it, but pain or no pain it's going to happen. Feeling that dog in you is first and foremost in your mind despite the pain. The mere thought of her getting fucked by a dog should be intoxicating.