Auto store. There's usually at least something available.

You can check something like woodcraft stores, but they tend to not have many hammer/mallet options when I've gone in person, and I've never really looked at their online store.

You're still likely to run into having to adapt something, rubber as a mallet material has limitations, so they tend to be no smaller than two inches in diameter, that I've ever seen. Now, plastic, I've seen as small as a half inch head, but never rubber.

Now, there are gunsmith hammers with interchangeable heads, including rubber options, and they're fairly small, but maybe too small, depends on what you're doing exactly. Too light would be just as likely. But it's an option. Those are usually available under 40USD, so feasible for a one off job, but not as cheap as automotive mallets. And you'd need you look online for those.

Can you describe the use case? Might help narrow the field of where to go (I'm assuming you checked places like amazon and didn't find what you needed, otherwise, I wouldn't have focused on the options I did for where to look)