Yes getting cuttings or bits of plants from friends/neighbours is the ultimate in cheap gardening.
We have a local nursery which probably can't compete with the likes of Aldi/pound shops on prices but the quality is great and it's cheaper than a garden centre. There's also a couple of stalls on our local market which are good value.
Some plants self seed a lot so buy one plant and within a year or two you could have lots e.g. alchemilla mollis, grasses. You can also split perennials if you don't mind waiting a bit for them to grow. I've got loads of a particular white geranium that spreads a bit so I keep digging bits up to fill holes in other bits of the garden. Ditto a phlox.