I would run far away from buying an email list. It will be a bunch of people that don't know who you are, haven't given you permission to email them, and don't appreciate you trying to sell to them without any understanding of their needs.
You would be better off building an email list yourself, likely much smaller, but full of highly targeted customers and potential customers. Then, build a relationship with them over time through value-driven emails.
You suggested sending a card first. This is a good idea. The call to action on the card could be to sign up to your email list. The trick is, that you need to give them a reason to want to sign up. What value are you going to give them? Could be as simple as putting together a 1 page Cheat Sheet for caring for your lawn.
Essentially, they have then given you permission to market to them.
I suggest checking out "Permission Marketing" by Seth Godin. There's probably a summary online somewhere if you don't want to read the whole book. But it's a pretty quick read.