I called my petsmart with the same question. They told me to bring in the petco ad, and then they would match with it.
But, I've also heard that they can be difficult about it as well, just depending upon who the manager is. If it was me, I'd call first, get their name, and THEN go to the store. That way if they question, you know who you talked to, and they're more likely to honor the match. And, if they tell you no on the phone, then you haven't wasted a trip.
Here's a link to the petco ad:
Just hit that button and put in your local zip code, and the flyer will come up on screen and you can print it out.
For what it's worth, here's a copy of my local circular:
(just click on the upper right hand corner to "turn" the pages, the sale tanks are in the back pages)
Or, if this doesn't work for you, I'd bet if you went to a petco they'd give you an ad. I was a pharmacy tech for many years at CVS, and we always had piles of the sales circulars lying around, I'd assume most retailers are the same. :)