I've been using this library by nostr:npub1g53mukxnjkcmr94fhryzkqutdz2ukq4ks0gvy5af25rgmwsl4ngq43drvk https://github.com/mmalmi/nostr-social-graph

Its nice how it has an export and import format. and it looks like there is also a server that could be used to provide pre-built graphs for bootstrapping https://github.com/mmalmi/nostr-social-graph/tree/master/server