When in neighborhood view, you should see a few blue buttons at the top left of your screen.
Click on on of those and you should get the option to place a pet store.
Or, click on the house bin, just below the button you click to create a sim. Scroll those the bin and you'll see a pet store. Just hover your mouse over the choices for a moment.
ETA: If you still have questions, here's a step by step
guide to help you out.
All I did was Google: The Sims 2 Pet Sore.