Goksular Yapi Market / B&Q, next to the Citroen dealership and the shop on the opposite side of the road that sells chain-saws, mowers and rotovators, sell packets of seeds. Many of the seeds that B&Q stock are Johnson's and are cheaper than the UK price.
If you turn right at the lights after Migros and head into town there's another shop about 50 metres along.
There's also a little shop behind the "everyday fruit and veg market" in the centre of Fethiye.
The Tuesday and Friday markets in Fethiye and the Saturday market at Karachula usually have a good selection of young vegetable plants throughout the year.