I love bootstrapped companies. I was in the exact same position about 6 years ago. My advice: it is time to put a grey hat on. I'd do the following:

1) Buy an email lists of property companies and send some unsolicited emails to people that want your product but don't know it . "Are you tired of dealing with rental application forms?"

2) Use fiverr to contact property owners on craigslist.

Once you get to about $10k/month you can stop doing these things and there will likely be natural organic growth. You can then start spending a few grand a month on "Free Rental Application", "Rental Application Template", etc. keywords. That will drive predictable growth over time with very little effort.