With regard to the original question,
i.e., whence did some of the Grand Army of the Republic's heavy equipment come, the following information could potentially be of great interest... and yet is most probably completely unnoticed by most observers:
ZOGOTO, EXARGA - Despite repeated assurances that they do not know who authorized the strip-mining of a doonium-rich asteroid belt in their sector, the Exargan Mining Authority has been fined for failure to disclose revenue to the Mining Guild. The Outer Rim world administers a rich asteroid belt deep within its sector, which is used to supply such major consumers as Kuat Drive Yards, Rendili StarDrive and Rothana Heavy Engineering. Their year-end inventory revealed a discrepancy of over 200 million tons of doonium ore. The Mining Guild is poised to launch a full investigation for corruption within the executive board of the Authority. In the meantime, they are being fined an amount based on the current market value of the doonium, well into the trillions of credits. [HoloNet News Vol. 531 #52: "Mining Guild to Fine Exarga"]
What is of particular interest to this excerpt is the fact that 200 million tonnes of doonium ore "mysteriously" disappeared at some point the year
before the events seen in
Attack of the Clones, and that the discrepancy was only noted in the months immediately prior to those events.
What is of even more interest is that the Exargan Mining Authority, the mining interest which failed to account for the said missing ore, was a supplier to Rothana Heavy Engineering, the dummy corporation for Kuat Drive Yards which manufactured the
Acclamator-class trans-galactic military transport ships, AT-TEs, SPHA-Ts and LAAT/i gunships seen in use by the clonetroopers in
Attack of the Clones.
Although it is not certain that Rothana came to possess the said missing ore, if one accepts the likely, implicit connexion between the two, it provides some idea as to when Rothana began construction on whatever war machines require doonium.
On an unrelated note, this is the second confirmed reference to the Mining Guild in the prequel era. The Mining Guild is as of this time the only corporate co-operative known to have survived the Clone War and to still exist in the time of the Empire. Given that Bespin's Cloud City was too small an operation to merit notice from the Guild, there exists a mostly useless indication of scale of the Exargan Mining Authority.
Publius