Oxbow has a line of adult rabbit pellets called “Natural Science Adult Rabbit Food” that lists a minimum of 3% fat (most quality pellets I’ve seen list a 2% minimum). I’ve been using this for my senior bun for at least a year (maybe longer) as one more tool to help keep his weight up. It also has a low (but acceptable) calcium level, which I pay attention to because of a previous bladder stone condition.
Here are some product links:
http://www.oxbowanimalhealth.com/products/…bject=8239
http://www.amazon.com/Oxbow-Animal-Health-…B007PZEBTY
I get mine from a local PetSmart (where it’s actually cheaper than the Amazon price from the above link).
I don’t know how old your rabbits are, but I’d be cautious about feeding anything to an adult on a regular basis with a higher than recommended calcium level. Bladder stones and sludge are not uncommon, and trying to increase weight with a food that’s high in calcium may solve the weight problem at the expense of other problems later on. If you have a rabbit-savvy vet, I’d talk to her/him to get some recommendations.