Alkalinity is raised by adding buffer. There are several good brands you can use. I perfer using the solid buffers that you mix in your own water. This may not be as good for you since you are buying water.

The trick to adding buffer is to add it very slowly. You can drip it in slowly by siphoning it into a high flow area using some air tubing and a little plastic air valve to regulate the drip rate. The key is to test before and after adding the a certain amount of the supplement. This allows you to figure out how much of the supplement raises the levels in the tank by whatever the difference between the first and second test readings. This information can be used to tell you what a certain amount of supplement will increase your levels.

I would also suggest you do a lot of testing in the beginning to see how fast your levels decrease in a day or a week. This will allow you to determine how much additive to add per day or week to maintian the levels where they need to be. Alkalinity does drop fairly quickly and you may need to add buffer a couple of times a week.