R.R. Bowker, LLC , a.k.a. MYIDENTIFIERS, is the primary place independently published authors buy ISBNs. The company announced on November 1 that its ISBN site has been breached and hackers have stolen credit card numbers and made unauthorized charges.
Bowker says they are disabling purchase of ISBNs while they are investigating this and developing security enhancements.
Thanks to David Colin Carr for alerting us.