You can't buy them everywhere. Huppin's might have it, their site lists it. You can't check to see if they have it though, and you can't order online.
Not much cheaper then the Olympus battery, if you buy the Olympus battery on Amazon or B&H.
But if the local store has it, you get it sooner. It's also possible to get a counterfeit battery from Amazon. So you might as well save the money and buy the third party.
If I get a counterfeit battery from Amazon, and it swells up, will Olympus remove it?
Most third party batteries lie about the capacity. There was an article I found on Google, didn't read all of it though. Don't know what the Promaster capacity claim is, or the official Olympus, and to lazy to look it up.
The Olympus is 1210 mAh. The Promaster doesn't say how much it is, just "Replaces: Olympus BLS–5/50 7.4V 1400Ah". There's no way the Promaster is 1400Ah, you'd probably have an explosion in that case. If they mean mAh, that's more then the official battery, therefore a lie. 1400Ah is 1400000mAh. I'd like to have what Huppin's is smoking. Or not.