I got edging for my borders in Aldis it is a strong small green fencing which inserts into the ground very well.
The only issue I have, is when mowing the lawn, very difficult to get a clean finish as the grass is growing up along the fencing, and for some reason seems to grow faster.
What I find myself doing is removing the edging, and going over it with the lawnmower for a clean finish..
If I was doing my garden again, I would raise my flowerbeds from the grass level... and have a clear gap between the grass and the flowerbeds, to make it easier mowing the lawn!!
To answer your question, Garden centres would be your best bet. I know Argos have a garden section, but if i remember, they only had the log rolls!.