One very good place to start would be here:
Free Shipping - ISBN: 9780890245422 - Paperback - Kalmbach Pub Co (edition ) - 1979 - Condition: Fair - Ship within 24hrs. Satisfaction 100% guaranteed. APO/FPO addresses supported - How to Build Model Railroad Benchwork
www.abebooks.com
The author, Linn H. Westcott, pretty much pioneered the L-girder idea, so you could say he literally "wrote the book."
Linn was also a well known and still well remembered editor of Model Railroader Magazine, assuming you need any other references to his pedigree. IIRC, he was also a pioneer in building the original "momentum throttles." I built one of those myself, and it still works well today (although I should probably check for capacitor leaks, come to think of it).
If I also remember correctly, Linn's original L-girder concept used typical dimensional lumber--1x2's, 1x3's, 1x4's etc, which usually had to be left to "season," to take out any warps. That was recommended anyway, although building the L's themselves do reduce the need for that. I think it's a lot more common today to build the L's out of ripped lengths of plywood, which is a lot more consistent in dimension and stability both.
I do recall too that there were, and maybe still are a couple of sources for DIY kits and parts for L-girder construction. FWIW.