Open Loads
The NMRA" Bulletin" , which is of limited use to be sure, spent a great deal of time beginning in around 1995 presenting a thorough selection of common gondola and flat car loads.
If you can attend the next Great Scale Show at Timonium, Md.- coming up now- you will find several people selling them . One gent sells MDC cars and loads he makes at quite reasonable prices.
Many European firms make a variety of excellent gondola loads that appear on their websites that you can buy, or use as prototypes for your own creations. This is a much larger section of the model railroad business than in North America , where their are few producers( like Jaeger, in wrapped lumber- they tend to concentrate in given areas).
M+D produces many steel industry loads, as well as more general loads that include the best sugar beet loads available. They are online and sold in U.S. by Miami Trains, among others.
HEICO (a/k/a Ladeguter Bauer) has large collection of loads on
www.
Relatedly, the entire manual, from the US Army for loading materials a/ks in open railroad cars , and securing same, is available on line, as a download, from the U.S. Army at Carlisle Barracks.
An alternative is using containerized loads commonly handled in gons as well as flats. One seen near ports, and not produced domestically, is coil and roll steel containers produced only by Broad Gauge Bodies in 'HO'. They have wwws, but I would recommend going through Lloyd's Model Railways, the large maker/retailer (like Walthers' only Australian) if you do usual "http://www." and put in next "lloydsmodelrail.com.au" that should get you there.
Using GOOGLE, or similar comprehensive search engine, you can find these sites. The URLs are too long to fit on single line in the format used on this site . Another site you might want to look up is the German model railroad manufactures site list. This gives all of them , and indicates their products and scale. It also has some Swiss , and eastern European non- German makers that gives you access to about 20 or so open freight car load making firms with www pps. , (and more without wwws)
Good-Luck, PJB.