Almost as soon as Meccano was introduced, the parts were sold separately. At least in England, there appears to have been significant sales of "spare" parts. For example, my Dad's collection of Meccano almost certainly started as an outfit (or perhaps more than one - my Uncle has a collection, too). It apparently was not long after receiving the initial outfit before he started using his pocket money to buy additional parts. Dad told me that he would select a model he wanted to build, and then buy (perhaps over a period of time) the parts he needed for the model. The models were often selected from his collection of Meccano Magazines, where interesting models were illustrated and described, and where a parts list could be found.
This traffic in parts was encouraged by Meccano, and they provided cases to make it easier for shops to store and display the parts. These cabinets eventually came in several sizes, and became quite large as the range of parts grew.