There are also very small tanks that can be set up for one (or very few) fish, such as a Betta fish. Bettas can even technically live without a filter system, since they can breathe air, however, you wouldn't want to leave it for a couple of weeks. When I was a kid, I had a small plastic tank with a Betta and a few Zebra Danios, with a cheapo corner air filter, and they did just fine.
If you go through with getting a tank, be sure to check the ideal temperatures for your fish as well. Some species are cold-hardy (think typical office temps), while others may require an in-tank water heater. Best of luck!