Another option might be to download the entire .com registry database from ICANN and query it live. I say “might” because I am not sure if your use case would be permitted under their TOS.
Yup, someone's suggested that. I might do it going forward, if it's allowed, but for the first day I tried to prioritize getting it out there.
Also, I have another project for which I store a huge amount of data, and as it turns out, data costs a lot of money to store! Depends on the size of the list, I might be better off paying for the Domainr API.
Just checked, latest zone file for .com is 4.9GB compressed, .net is only 456 MB compressed (2.2GB decompressed). Not sure how large .com is decompressed as CZDS is giving me download issues for .com today but some rough extrapolation from .net would land around 24GB.
I will try it in the next few days and report back. Most likely a combination approach would be the best - check against the known data, then double-check with API on case-by-case basis. I might require login to get extra data, so it would be just a subset of users doing a deep-dive. Thank you!