The only way to get a fractional share is in a divdend reinvestment plan (DRIP) but you must own one share before you can enroll in a DRIP. There are plenty of companies who have DRIPs but not all and companies that do not issue a dividend will not have a DRIP and not all companies that do have a dividend offer a DRIP. I do not know how share builder works so maybe they sell fractional shares of any company. The way a DRIP works is the company issues you shares instead of cash so say you get $10 in dividend for XYZ and it traded at $50 you would receive .2 of a share instead of the cash. The fraction will go to four decimal places and in some cases the fractional portion of your account may not receive a dividend it depends on the plan but if you liquidate your shares the fraction is counted.
I hope this helps
