I can't say I've ever had a problem finding barrel length. The only manual I have that you can't find a barrel lengths in is Lee's Modern Reloading, whose data is mostly compiled from other manuals, so all the data under one chambering isn't necessarily from the same length barrel.
You can usually tell pressure/velocity barrel data from the other data by whether or not pressure numbers are listed with the loads. If there are pressure numbers, it came from one of these test barrels. That includes all Hodgdon's data (click on the "Print" button to see barrel length, case brand, and primer used), Western's data (Accurate and Ramshot), and Alliant measures their loads and does contract pressure testing for other companies.
In Lyman's manual it is mixed. You'll find all the loads that list pressures turn out to have been fired in universal receivers (the kind the test barrels are mounted in), but where there are no pressure numbers they list a production firearm. They always give barrel lengths.
Speer is a funny case in that they develop most loads in production firearms, but Alliant then measures pressures for them for liability purposes, though they don't list that pressure data. They do list some loads developed in universal receivers, but without pressures being listed. They do list firearm and barrel length for every load.
Hornady and Sierra just develop loads in production guns, as near as I can tell, but they list the guns and barrel lengths.
Commercial ammunition is always developed in pressure barrels because they use bulk powders, for which burn rates vary too much to follow simple recipes. It's also the SAAMI standard, so I'm sure there are liability concerns. So you can find the barrel lengths used in SAAMI's standards, where they list the test barrel dimensions.