I know the programme and made some for my wife afterwards (the clip's not working as I write this). It's not actually reinforcing bar you need however as that usually has a serrated profile. You just need round steel bar in 5-6mm diameter (depending on the size of support you want).
Try a search for e.g. "steel round bar " or "bright steel round bar " etc.). Here's the place where I got mine to give you an idea of what kind of company they are:- http://www.pyramidsteel.co.uk/index.php?webpage=Bright_Steel.html
These are trade businesses but were happy enough to cut some to length for me and supply.
I have a feeling it came in long lengths and I needed to have them cut down to size. On their site now it says 3m lengths which sounds shorter than I remember. I want to make some more now and will buy longer lengths than before. By the time the curve is included, you need them longer than you think. I have a feeling I bought 2m lengths, but I would probably go for 3m now. That would make something to support quite tall and large groups of plants.
You can work out what length you need like this. Take 2 x the height of the support (including how much you want to go into the soil). Then measure the diameter of the dustbin or whatever you will be using to form the curve of the support.
The length of bar you need for each support is (2 x height)+(diameter x 1.57). So for example if the height of upright you want is 80cm and the diameter is 90cm, the length of bar would be (2 x 80)+(90 x 1.57) = 301 cm or ~3m.
This assumes that you bend the curve to form a semi-circle, but the process is a bit less precise than this and you may bend less of an arc.
I'd reckon on 2m length for a smaller support bent around say a large plant pot and 3m for a large support. We had a handy terracotta urn water feature which was perfect for bending the curves (stays put more easily than a bin or whatever).