Adwords, Forums, facebook groups, blog audiences, commenting, twitter, pinterest. Go and find out where you customer hangs out online. Who already has your perfect audience on their mailing list? See if you can write something of value for their audience and tap into existing groups. When you do get users, ask how they found you then double down on promoting in that channel.
Some of those suggestions (eg. comments) may represent a risk to search ranking. From what I understand, Google considers comments a negative signal. Perhaps it's only duplicate comments, though. Can someone more experienced than I am please provide their perspective here?
Also, if your customers can find you without the assistance of search rankings (which can change at any time) then surely that's a good thing.