You can enlarge your storehouse at the Exchange Market, "storehouse and sales at the black market" and press a square button "Increase the storage capacity" there.

The price of the stock increasement is:
Lev 1 and 2 = 2500 L$
Lev 3 = 5000 L$
Lev 4 and + = 50000 L$.

The sizes of increasements remain always proportional to the level.

To have more space, you can always use a paying option of several enlargements for 1 code in the Super STOCK Service... which remains optional.