Fantastic find, making me pretty jealous. Been searching for one for ages!
They're definitely hard to find. I found all of mine searching on the net and contacting other collectors of antique aquariums. I've never stumbled across one in an antique store, antique show, or yard sale. They're just too rare due to the fact that the early ones were only available to the wealthy due to their high prices, so few were made. Many were melted down as scrap during the first two world wars. So you're left with very few survivors.
I have a friend who is a prominent antiques auctioneer and has been making his living doing this for decades and he said he has never come across an antique aquarium or terrarium.