The smallest unit Sams sells is a 24 pack of bagged dichlor. The bagged material will keep for a couple of years, so it's appropriate for a very small pool like yours.

The price varies around the country, but here it sells for about $2.50/lb or $60.

However, with a really small pool, using bleach before and after each use, may be a better idea. The problem is, without stabilizer sunlight will remove ALL the chlorine in as little as one hour, no matter how much you add. Your PF is 75 (your pool has 1/75th of a million pounds of water) so 1 pound of stabilizer will add 75 ppm of CYA to your pool.

A gallon of 8% bleach contains about 0.7 lbs of chlorine gas equivalent, so an entire gallon would add 75 x 0.7 or ~50 ppm. That means 1 cup of bleach would add 3 ppm of chlorine. That's a reasonable dose. If you add 1 cups BEFORE and 1 cup AFTER each pool use, your pool should stay in reasonable shape.

One more thing: stuff keeps trying to grow in your pool even when you don't use it. So . . . add 1 cup EVERY evening, whether the pool is used or not. That way, the chlorine can remain overnight (no sun!) and clean up whatever's in the water.