[Careful]

It really comes down to the amount of risk you're willing to tolerate. There's definitely a risk that a non-aquarium-designed ornament could leach something into the water that kills your fish. There's also a much bigger chance that you can put it in without any problems. I have a painted-resin buddha in one of my tanks that has been in there for ~8 months and none of my fish have suffered from it even though the paint is starting to wear off. It's really your choice. And it's not like some governing fish body is overseeing the manufacture of every single aquarium-designated decoration anyway so there's a chance (however small) that even something you buy at Petco will leach something that'll kill your fish. Good luck!