I did the same in my garden a few years ago now. I went to my local garden centre and bought a few pots of thyme making sure they said they were a "carpeting" variety. As Berghill says, some of them can grow more bush like. I split each pot into a few small plants and now they have spread together filling the spaces between the stones. Mine are a mix of colours, purple, pink and pale pink almost white. I will have a go at uploading some photos if I can.