I don't know what works with the Born Free bottles.. but we used Dr Browns bottles (they were supposed to be the best for gas, but they were a pain to clean). They have a standard size nipple in level 1, 2, 3 and a y-cut (for rice cereal thickness). If your bottles take a standard size nipple, you could try those. We were on AR for a while (it only helped for a couple of weeks and we had to switch again), but I think we used a level 2. If I remember right, the AR formula is only slightly thicker as they are drinking it, but it thickens in their tummy so you wouldn't want to go too fast of a flow.
I'm pretty sure there are quite a few brands that have the different flow nipples...