Ken,
Do you want I-beams that are as long as the bridge itself for spanning the gap on the layout (video)? I take it this is 3-5ft? The closest i could get to something similar is some lengths made of either steel / aluminum / brass (i think) from a home improvement store locally*, but they didnt have i-beams, just squares, angles, rods and rectangle profiles. Having said that a rectangular profile with added styrene caps could be made to look like an i-beam. I hope i am making sense with my description. On a different topic, i have seen ready-made metal bridges similar to the one you posted that have a span of 2ft by hack-brucken brand. Maybe these could be of some help at least as a base for the project.
*I recently bought some angle-profile aluminum length in order to brace internally a big kitbashed building i am constructing.