Flabbergasted rhymes with:
forecasted, outlasted, sandblasted, attested, bested, blacklisted, boosted, busted, coasted, costed, crusted, delisted, detested, digested, disinterested, divested, dusted, encrusted, enlisted, entrusted, fasted, feasted, fisted, foisted, forecasted, guested, gusted, halsted, hosted, husted, lambasted, lusted, maladjusted, mistrusted, molested, ousted, outlasted, pasted, plaisted, posted, preexisted, readjusted, rearrested, reinvested, requested, resisted, retested, roosted, sandblasted, tightfisted, toasted, trusted, unadjusted, unassisted, uncontested, unlisted, unmolested, untested, wixted, worsted, wrested
Flabbergasted sounds like:
flabbergast, flavorist, fulbright, fullbright