Jean
I don't know where you might buy them, I'm not entirely sure that they actually make nipples and cuffs. Maybe you could try buying a feeding system that you know well and trust. I used the Drop-Ins for my son, they were great, but they did cost. I use the Avent bottles now for my baby girl (on the rare occasions she takes a bottle, that is!), and they're even better. In general though, there are two types of nipples and rings - a wide nipple and a narrow one. The sizes are generic within those two bands, but some brands, like the Avent, only make nipples to ft their own bottles.

Mackenzie goes BLEHHHH
I don't know if I've ever seen them in the store. I only got them in the hospital EDIT: I just googled it, and found them online

KLynn
For some stupid reason, you can only get them in the hospital. When I had my son I asked for a bunch. Then it turned out they don't last very long, they collapse easily, and baby gets too much air. I also found that most rings are pretty universal. I had the similac bottles you're talking about, medela breast pump bottles, plastic gerber bottles, and parent's choice bottles. The tops for ALL of them were interchangable. After all of that, I ended up getting rid of them all and switching to Doctor Brown's. They are the best bottles I have ever used. But if you really want rings and nipples to fit those similac bottles just go buy the parent's choice or gerber bottles. They come in packs of 3 at WalMart for less than 5 bucks.

Mung
I try to find for you and I found this... And... I hope this was helpful for you. Good Luck!