Regex,
As a proposition with no evidence to support it:
HAMMS was a US government project. The plans' intellectual rights belong to the US government. Unless The Smithsonian Institution has licensed a third party as a sales agent, anyone else selling those plans is infringing a copyright. I doubt that any legit alternatives to the S.I. exist.
To kibitz:
I remember when HAMMS plans were offered for sale. I was tempted. It was a crate of bound folios. It was expensive. It is also outside my focus era. Separating the plans from the binding would be burning money. Unless going crazy and buying a large number of plans - it is more economical to pay The S.I. $10 + shipping for each desired sheet and pay Office Depot the $5 per sheet to scan and save the PDF to your USB stick. With the way I work, there is no part of any plan that I would work as an individual component that is too large to print out on my Ecotank.
The present S.I. process brings to mind: obtuse, passive aggressive. Then again, should S.I. scan their archive of plans to PDF/TIFF and offer direct downloads on-line, it would make too easy for pirates and short sighted morons to destroy their income stream.
I hope S.I. has done scans and has them stored off site. Otherwise, if what John Saxon said: "There was a fire." does occur the loss would be beyond imagining.