The site hasn't been updated for years. I was thinking of trying a "domain broker".
Has anyone had any luck using these kinds of services?
I wonder if there is any other way I could contact the owner.
In the vast majority of domain transactions I'd suggest using a broker would be unnecessary. However, if you are in the hunt for what might properly be described as a "top shelf domain" ($100,000+) you might consider working with an experienced person.
Ways to contact current owner:
Use Archive.org to see if the contact info for the website has changed over the years.
Using a broker? Why do you think you need someone else to act on your behalf? Unless it's a website/domain that you are prepared to pay >$2,500 I'd keep it simple and suggest you handle it yourself. Alternatively, if you're not comforable "doing business", then I'd solicit a competent friend to handle the negotiations.
Anyone can act as an intermediary - a "broker" by another name - so long as they are trustworthy, experienced with the subject matter (domains) and have communication and negotiation skills.