Start with an npub that has a large follow list. Query its follow list (Kind 3), and for each pubkey in the list, query its Relay List Metadata (Kind 10002). Then, recursively repeat this for followers of followers, and if you're brave enough, go for level 3 as well. As you traverse the graph, store all relays from Kind 10002 in a set. You’ll end up with well over 1,000 entries in this set. Although you’ll find a lot (and I mean a lot) of junk. Clean up the URLs, test the relays, and Bob's your uncle.