The resin bases (grey ones) are bought off ebay, around 40p each from Fenris Games
The others are a mix. Regardless I have taken a standard base and levelled it off with wall filler or equivalent. Once dry I level it out. Then either top with fine sand and paint as ash felt, or I carve, with a modelling tool, some lines to look like paving stone, cost is minimal per base.
Alternatively I have used my electric modelling drill with a engraving attachment to produce irregular flag stones, but this is perhaps more useful for basing dungeon creatures as it creates an irregular look.