This is for free.
Chose your area by Country and City
Since your blog is all about pets create a section to find a vet/petshop in your area. (go from smal to big on this, otherwise it will consume all your time )
Example Find a vet o a small comunitie
After you create all pages you need to start to find the vets/petshops on that small comunitie, then you move to other comunitie or town on the same city, untill you complete the city, then you move to other city.
At the start will be complicated, you will get bored, and lost, so use google to find all the info from the vet or pet shops arround you.
After a wile you will have a great directory to find a petshop or a vet (in a hurry can be very usefull)
After that you can start to plan how to sail ads to other related busines on your blog.
Here you have an example of that...
http://gyms-in-cork.blogspot.comI just start that blog a year ago and becomes the number 1 for a lot of keywords, and it sail some ads, i don't live from what i get from that log alone, but is funn, and i get some cash.
Remember,
Set your priorities
Set your goals
Put all the info for free until your blog get there, once your blog is visible you can start to sail ads on your blog.
If you sail the ads on your blog, you will see that you get more money from that, compared to what GoogleAdsense pay in some cases.
Sorry for my english