The thermal printer that was sold for use with the Mattel Aquarius or VIC-20 / Commodore 64 is an Alphacom VP42.
This uses thermal paper, 4⅜″ wide. Original paper for these printers is very hard to find. Thermal paper however is still being used in cash-registers, but the 4⅜″ format is not very common. So I wonder if anybody ever found a replacement?
Fax machines used to use thermal paper by the pallet, and it is still commonly available wherever office supplies are sold (including amazon). While this will be wider (not, as the problem is with the cash register rolls, narrower) than what these vintage printers need, this problem can be fixed with a guide and a sharp knife (and in the worst case re-rolling the paper onto the core of an original roll if there are issues with core diameter...).
4 3/8 inches is 111.125 mm.
A Google search for the Swedish term termopapper (thermal paper) 110mm gives me at least four hits on the first page alone for online stores that sell such paper. While none of those four appear at a glance to state whether it's in stock or not, it's clearly possible to procure. They even appear to offer different core diameters, so it shouldn't be too hard to find one that will fit.
That would be the same type of paper as is used e.g. for cash registers (at least one of those sites offer "not sales receipt" card terminal rolls alongside), and I can see no good reason why it wouldn't work in a printer designed for thermal paper. At worst the print quality may suffer slightly, like how inkjet printers need specially coated paper for best quality results (but will work with almost anything resembling paper of appropriate weight/thickness), but using generic thermal paper shouldn't cause any hardware damage and should certainly allow you to use the printer.
In general, I would expect any decent office supply store to either have it in stock, or at least be able to order it for you easily enough. Just don't mention that it's for a retrocomputer printer, or it will likely get a lot more difficult. Office supply store salespeople are used to dealing with customers who know exactly what they need, so just asking for rolls of 110 mm wide thermal paper should be enough.
