Reddit reviews A Modern Manual of Smallsword Fencing

Here are a couple of modern secondary sources which might be of use:

https://www.amazon.com/Newer-Shorter-Easier-Method-Fencing/dp/0956487165/

https://www.amazon.com/Modern-Manual-Smallsword-Fencing-ebook/dp/B00TIWMIYM/

Here's a good list of original treatises:

https://smallswordproject.com/historic-texts/

Modern sport fencing will certainly teach a lot of what you need for smallsword fencing, but it will also teach some very bad habits. If you can't find any means of getting smallsword training then by all means take it up. Avoid "sabre" and pistol grips, and take a look at the foil section of Hutton's "The Swordsman":

https://www.amazon.com/Swordsman-Manual-Defence-Against-Uncivilised/dp/1845743652