New I beams are 25 ft long @ approx $175 each. You'll need a piece of 4 inch channel at approx.$100. Everything cuts with a chop saw and bolts together.
When I was considering cradling my cat, I called around and Lou's Marine in GB had the absolute lowest price (by up to 50%! in most cases). Give Gary Fagin a call (he owns the place) and tell him I referred you.