There were Resistance supporters known since the time of TFA (they were the ones who donated new model X-Wings, for example). From Starwars Databank article for X-70:
The Incom T-70 X-wing is the latest incarnation of a classic design, and the signature combat craft of the Resistance’s Starfighter Corps. Faster than the Alliance-era T-65s, Resistance X-wings are maneuverable enough to engage TIE fighters in dogfights, but powerful enough to take down capital ships. The credit-strapped Resistance relies on X-wings donated or lent by sympathetic New Republic senators and local security forces.
Sure, the Republic center and leadership are gone but the systems those Senators were from, as well as those "local security forces", are still around.
We don't have any canon specifics beyond that, however, until maybe novelization comes out.