Can't vouch for what's on directory.io but it goes as far as http://directory.io/9046256971665327767466483203803742801002... & beyond... have you tried paging through?

https://en.bitcoin.it/wiki/Private_key

"In Bitcoin, a private key is a 256-bit number, which can be represented one of several ways. Here is a private key in hexadecimal..."

Edit : The bitcoin private key being a number was in response to your redacted comment.