I've found in most of my conversations that unless the seller explicitly tells them that they're willing to carry, it's difficult to find out. The agents I've spoken with tend to either not understand it or they're just unwilling to ask the seller. It's unfortunate.
However, maybe 10% of the ones you ask would know/understand and maybe 10% of those would be willing and able to get you a seller financed deal. So it may take you 100 deals to look through to find seller financing. Maybe it's less, maybe it's more.
I would think (theorizing here a bit) that if you're able to find a deal you'd really like to have and talk to the seller and find out their equity position, you might be able to talk them into it because of the tax benefits and the ability for them to get more money than being bought outright. Maybe they've held the property long enough that it's paid off and they're just wanting out, but don't want to give up the monthly income or take the tax hit on a fully depreciated property. Could be a good opportunity then.