Don't even go by gallons. The tops I've seen for sale don't even do that. They are sold by dimensions because that's the only thing that matters. The top doesn't care how tall, or how much capacity the tank is. For example, a common standard 10 gallon tank is 24X12 inches and a 20 gallon long (like the reptile tanks) is 30X12 inches but if those tanks were taller, and therefore more gallons, the tops would still fit. I don't know who else sells them, but petco has clips for just a couple of dollars that hold these tops on. You cannot get by without them, or else you have to put something heavy on both ends of the top which sucks for placing lights on top.

Here's a useful link for tank sizes. Comes in very handy if you want to know how many gallons a tank holds. As long as it's not custom, you measure it and find the gallons here: Chart of Tank Sizes

Not that us reptile people care how many gallons it holds. What mainly matters is floor space dimensions. Don't tell mom "20 gallon" tell her 30X12 inches. They're way cheaper online though. Even after shipping they are only half as much money as pet stores charge. Petco charges $25 for 24X12 top. Online they are about $12 shipped.