Researching Market - Finding Your Target

I have a gardening site which ranks well and gets decent traffic. I am adding a directory to the site in order to:

1) Give my visitors resources to visit, call, etc
and
2) [obviously] produce more income for the site

My question is this: What is the best way to go about gathering an email list to invite businesses in the categories I am carrying to join the directory? I plan to offer this initial group a discount by giving them lifetime links and then once the dircetory builds up a bit I will move to an annual subscription rate. How can I find (or rather what's the most efficient method of finding) a large list of businesses related to my niche?

I don't want to purchase a database (haven't found one and these are often outdated).

Thanks in advance for any input.

 

perhaps join other gardening groups, such as yahoo/msn groups?

 

Google is your friend!

Also try posting on DP in the link sales section (say you have an opportunity for garden related site owners)

Some PR in your directory will be handy aswell