ACK! Please don’t do this. Bettas are no different than any other fish and require proper filtering and temperature control in their tanks. You may keep the fish alive for a little while, but trust me, your fish will get sick and die if it’s not in a proper fish tank.
Petco has a really nice 3 gallon tank (which is relatively small) that comes fully equipped with lighting and filtration (including a “bio-wheel” that will help balance the bacteria/biological filtration) that runs about $40.00. Here’s the 6 gallon version (doesn’t appear the 3 gallon one is available to order online, but you can get them in the stores).
Please believe me, it is worth the extra few bucks that you’ll spend up front to do it right than to waste tons of time and money on the 1 or 2 gallon tanks that only use undergravel filters (which are NOT sufficient), which you will end up throwing out because your fish will keep dying in them. (I know - I have 3 completely useless 1 & 2 gallon tanks in my garage from trying to go that route and failing.)
If your office temperature is not regulated enough to go without a heater in the tank (Bettas like it warm - at least 78 to 82 degrees!), you can pick up a small heater at Wal*Mart (7.5 watts) for $8.00. The thing to be careful about with that particular heater, though, is that it’s “pre-set” so there are no controls on it. It COULD overheat your water because it’s constantly in an “on” position. What I did to combat this is to buy a lamp timer at Target for about $10.00 and plugged the heater into that. I had to experiment a bit, but for my environment, having it go on for an hour, once every 8 hours keeps the tank at a perfect 80 degrees throughout the day and night. If a Betta’s temperature fluctuates too much, he will go into shock and die.
Take it from someone who learned the hard way, starting out with those Aqua babies that come in a little box (much like the cookie jar idea you’re thinking of going with). You will only get heartbroken or frustrated trying to keep fish alive in that type of environment.
A fish tank can be a wonderful addition to an office (I keep mine right next to my monitor), but your best bet would be to do it “right” right from the get go. Good Luck!!