I've been able to do all sorts of things with Imaginary Friend dolls with the help of mods. I think there is a mod at Mod the Sims that might allow you to buy one but I tried it and haven't ever been able to find out how it works.
If you change the chance of getting an IF using Returner to 100% and have another baby, it will get an Imaginary Friend. Once there is one in the household, I use NRAAS Debug Enabler to clone it. Then I give it a different owner and change it's appearance all with Debug Enabler. Some of the actions are a bit tricky. It can depend on which sim is active when you're using Debug Enabler to do these things. I generally end up experimenting each time I try to do it until I get it to work.