I know Vestratto has that as a 'FAQ' item on their website (parts list to convert USA to full Anvil). AFAIK, only Vestratto sells it (unless you find a parts source elsewhere, and cobble it together.)
If the price to get the remainder of the parts to add to the USA kit outstrips a full Anvil, I'd just get the full Anvil and either keep your USA as a spare, or put it up for sale. YMMV.
Also, full Anvils are available from a very few US vendors, if you don't want to deal with cross-border shipping from Vestratto, direct.