To buy simply go to Money Records/Notify and state that you have brought a certain amount of Business Land or Housing Land. The value of the land in E$ will be taken from your Bank Account and your new Land blocks will be registered at Land/Records.
Once you have brought land you must build on it to actually have a house/place for your business. That can be done at the Building Agent. You can also add to your already existing house/business place's building there (if you are purchasing additional land).
The maximum amount of housing land one may own is 10 blocks, the maximum amount of business land one may own is also 10 blocks. So, in total maximum, a citizen can in theory own up to 20 blocks of land.