Unfortunately not every domain that has no DNS record is available for purchase: some people configure no records, others are reserved by the registry (many short words for nTLDs). If something isn't in the DNS zone you can do a live DNS and WHOIS check to be more certain.
A while back I tied together this DNS zone + live false positive check into a website (for all TLDs and with price info):
Congrats! The interface is just super neat, how it gets updated when you hover (I imagine it caches the information about that specific domain for the next user).
A while back I tied together this DNS zone + live false positive check into a website (for all TLDs and with price info):
https://domain.garden
AWS credits are still paying for this, so feel free to use it!