Li, I went up to the Great Wall Supermarket in Rockville last Sunday. They had small Marble Gobies in one of the live tanks. If you bring a bucket with water in it and leave it in your car, you could just ask them to put it in plastic, pay for it, bring it to your car, and then bring it home. (That's what Josh did with the weather loaches at H Mart.) They're small-- only about 8-10 inches.
Here's a link to one of the grownups:
http://www.lurehaven.com.sg/Galleries/FW/Ima/Images/IMA-Ligid-70-Marble-Goby-(3).jpg
They're so cool! If I had an indoor pond, I'd get one!