I've already emailed Audio Adviser asking for dibs on any working returned centers or bookshelves in the LS series, since they have been sold out of them for over 30 days that they will not be available through their site. fleabay, craigslist, audiogon or some other personal listing will be the only way to get an EXACT matching center, otherwise the previously mentioned Energy center would be your best choice, second from that would be a mirage or older-generation athena center speaker.
Edit: Mirage Omni C-150 is the exact same as an Athena LS-C100 AND the Omni 150 is the same class of speaker as an LS-100, same components but a different orientation.