If you want a toy with an inflatable knot that is more discrete than realistic ones or you are into veterinary medical play, you could drop "dog insemination" in the search function at aliexpress. In addition to various small stuff, you also find insemination devices or kits which are effectively silicone dildos with inflatable knots and a central channel. As it is typical for aliexpress there are multiple listings, at least some of which offer a choice of size (this one has six which looks like the entire range).
Of course this being a generic item from China, I have no idea what type of silicone they are made from.
Of course this being a generic item from China, I have no idea what type of silicone they are made from.