Not much help with the I-beams, but this is a great site for supplies to make your own parts (not too hard):
http://uscomposites.com/
They've also got resins, brushes, gloves, mold material (rubber, though you can use anything as a mold, including wood and metal), mold release (carnauba wax), gelcoat (big thing... adds a hard, smooth finish to the material), etc.
http://www.acp-composites.com/ is another great place to get stuff. They have pre-made parts, though I'm not sure if they're in the sizes you need.
It seems that, at the moment, it may be easier to use fibreglass and/or Kevlar, because there is an industry-wide carbon fiber shortage. There is (as I recall from a year or so ago) only one factory in the world making it, and the US government is taking up quite a lot of it for new combat aircraft (F-22, F-35). There are/were plans to build a second plant, or convert another to carbon fiber manufacturing, but I haven't heard about it since I stopped doing model rocketry as much.
As far as using fiberglass goes, you want to use the matte material (unwoven) for most everything, and woven stuff for reinforcement. Also, use true epoxy resin (Aeropoxy, West System), not polyester. And try to vacuum bag if you can, as it removes air bubbles. If you can put it on a sheet of balsa (a couple of layers per side on a sheet of 1/16" balsa, for example - you may need/want more fiberglass or wood), you can get incredibly strong sheets, far stronger than wood, but just a bit heavier than a balsa sheet.
A note about using pure composite: Resin is heavy. It makes up most of the weight of a material, because there needs to be a lot of it. Make sure you have the weight before you use composite parts.
-Jake Alexander