real
leading
acknowledged, recognized
professional, qualified
self-proclaimed, self-styled
so-called
international, world
She is a world expert on butterflies.

local
independent, outside
computer, financial, gardening, health, industry, legal, marketing, medical, military, scientific, security, technical, etc.

committee, panel, team
A panel of experts will answer questions from the television audience.

ask, consult, talk to
seek advice from, take advice from (both esp. BrE)
hire

advise sb/sth, agree sth, argue sth, believe sth, claim sth, fear sth, predict sth, reckon sth (esp. BrE), recommend sth, say sth, suggest sth, tell sb, think sth, warn (sb)
Experts agree that a balanced diet is the key to great health.

expert at
He's an expert at getting his own way.

expert in
an expert in skin care

expert on
an expert on European art

Expert is used with these nouns:
advice, adviser, assistance, consensus, consultation, eye, gardener, guidance, hand, help, knowledge, opinion, panel, supervision, testimony, tribunal, tuition, witness