Some people free feed while others feed once or twice a day. It really depends on the person and the rabbits. I give pellets twice a day, hay as needed (usually at least once a day), veggies every other day. 3 of my 4 rabbits will just keep eating pellets, so can't have them all the time, they get a measured amount. My other ones eats throughout the day, but still gets a measured amount. I would give a measured amount for adults so you know what they are eating and can control the amount. Younger rabbits should get more than adults as they are growing, but adults really don't need a lot of pellets. My angoras each get 1/2 a cup per day and the others gt 1/4 cup per day (they are smaller too).
I think Petsmart does carry Nutriphase foods and there is 1 that I think is higher protein. I haven't looked in a while, so it might have changed or yours doesn't have it.
I would check feed or farm stores near you. Try calling them and ask what brands they carry then look those up online to see if it is a suitable product. The 2 brands that I do hear about for rabbits are Purina and Manna Pro. I can't get either brand here, so don't have personal experience, but I would probably got for Manna Pro if I had a choice.
When looking at the pellets, you should try to at least 16% protein, but not more than 18%. The fibre should be 18% or more, but a bit less isn't too much concern if you feed lots of hay. You don't want too much fat, I think 1.5% is good. The ingredients will probably have alfalfa first, I prefer to avoid corn if at all possible.