Be Helpful In Forums

Join forums in your niche and promote your site in your signature.

Be careful. Read the forum instructions first or you're likely to annoy forum
moderators and forum members who have been there for years. Lurk and
learn.

Some forums allow signatures. Some don't. Some won't allow words like
"See the link in my signature". Tread carefully.

You also need to remember that many affiliate agreements specifically say
that posting an affiliate link on a forum is regarded as spamming. Ignoring
this rule can get you dumped from an affiliate program. You could lose any
commissions you've earned.

Build your reputation by posting genuinely, helpful, useful comments.
If you think carefully of ways to provide truly USEFUL content that people
will talk about, you'll automatically end up with lots of free, one-way links to
your site as people recommend it without even being asked to do so.

Do a search in Google for
yourkeyword + forum
yourkeyword +"message board"