Can someone point me to any regulatory requirements regarding notification of IRA accounts (accountholders) that will be purchased from a bank by another bank specifically related to successor custodian notification? Are there certain timing requirements, if so, what? Any help is much appreciated.

This is covered in Treasury Regulation 1.408-6 (d)(4)(ii)(C): http://www.law.cornell.edu/cfr/text/26/1.408-6

(C) If the governing instrument is amended after the account, annuity, or endowment contract is no longer subject to revocation pursuant to subdivision (ii)(A) or (B) of this subparagraph, the trustee or issuer shall not later than the 30th day after the later of the date on which the amendment is adopted or becomes effective, deliver or mail to the last known address of the benefited individual a copy of such amendment and, if such amendment affects a matter described in subdivisions (iii) through (viii) of this subparagraph, a disclosure statement with respect to such matter meeting the requirements of subdivision (iv) of this subparagraph.