UPDATE 2-Ambac again postpones launch of municipal bond arm

* Unit CEO steps down

* Ambac to continue working on new businesses

* Shares down 16 percent (Recasts, adds company executive comment, background on Everspan plan)

NEW YORK, June 19 (Reuters) - Bond insurer Ambac Financial Group (ABK.N: Quote, Profile, Research) said it has again postponed the launch of its municipal bond arm Everspan after struggling to raise money from outside investors to fund the unit.

Ambac and other bond insurers had been pinning hopes for a revival of their business on launching municipal-focused units, after ratings agencies stripped the insurers of their "triple A" ratings following heavy credit losses from a foray into guaranteeing repackaged debt.

Everspan's chief executive, Douglas Renfield-Miller, had been discussing raising capital with third parties, but in a statement Ambac said the talks had failed to raise the funds and Renfield-Miller had decided to step down.

Investors were looking for higher returns than a municipals-only financial guarantor would be able to produce, said Peter Poillon, a managing director in Ambac investor relations.

"It just wasn't attractive enough," he said. "Eventually, the markets will come back down to earth and we'll market it again."

Ambac will most likely approach potential investors for Everspan again next year, he said.