Currently there seems to be only one company offering a high-quality string set for the 19th-century-style Russian 7-string guitar: Aquila. Recommended!
Available from their website in Italy: https://aquilacorde.com/en/shop/modern-instrument-string-sets/10-strings-classical-guitar/accordatura-russa-en/ or elsewhere.
Gut trebles are more authentic for period instruments. Try gut equivalents for the first three strings, with tensions 9.6kg, 8.1kg, and 7.5kg.