There are many different ways to get new plants, the RHS Gardening website has a good explanation of the different techniques.

Plant propagation does not need to be expensive and can reuse lots of items that would otherwise be thrown away or put in the recycling such as egg boxes, kitchen roll tubes, yoghurt pots, newspapers and old guttering. See Seeds

Other sources of plants are through plant swaps/sales and seed swapping. If you have an allotment there may be a place where spare plants are left for others.