How should I go about replacing my 2x6 garage rafters?

I have a 20 foot wide garage with 3, 2x6 rafters that span that length. There's plywood on top for storage, however since the house is over 40 years old, they're starting to dip in the middle. There's a couple of supports coming down from the roof joists to help hold them, but the bow is getting pretty bad. I want to replace all three and create a much sturdier platform up there for my storage. I can't buy 20 foot 2x6's, so could I create my own multi-lam beams or is there a better way to do this?