Once you own a stock, you can purchase more shares by using a dividend reinvestment plan (DRIP). A DRIP lets you buy shares in the stock for no commission charge, and can also let you reinvest your dividends into company shares instead of taking cash again at no cost.
Some companies allow you to enroll directly, most often from their corporate website. (Check the "Investor Relations" section of company site.) Others require you to already own at least one share in the company, and to own the shares in your own name instead of in your broker's name. Check out the requirements and see if this type of investing works for you if it does, you can cut future commissions on purchases of that stock to zero.