One type is incredibly simple. It is just a piece of plastic tube moulded onto a flat base. You simply put it into your plant pot and slot the cane into it. It has the blessings of simplicity and cheapness (25p each), but the tube is narrow (internal diameter approx 15mm) and will only accommodate thin canes. It will probably be good for supporting relatively small plants - such as chillies. You can get these from Gardening Naturally. On their website the price is shown as 30p each, and you can order singly if you want to, but at the checkout the price is adjusted to 25p each.
The other type is more like the items I have been using for some years now, but smaller. It is supplied flat-packed (in sets of 3), and to assemble it you just push the end of the vertical piece into a socket that sticks up from the triangular horizontal piece.
This type is designed for use with Growbags. The idea is that the weight of the Growbag sitting on top of the triangular base steadies the vertical piece sitting above it, into which you slot a cane.
I don't use Growbags, but this type of device fits well enough in the base of a large container.
On one of mine the socket is badly welded on (i.e. not vertical), which will make achieving a good position that much more difficult, but at least you can swivel the vertical piece in the socket in order to get it into a suitable position in the base of the container. The internal diameter of the rings on the vertical piece of this type of cane-support is marginally larger (approx 18mm) than the plastic sockets on the other type, and will therefore accommodate slightly bigger canes.
The problem I forsee though is that these new items are much smaller and flimsier than my older ones, and may not properly support the weight of a big tomato plant laden with fruit. Here is a comparison on the two:
Maybe these days people grow smaller tomato plants in their Growbags? That photo also shows very well how the new one doesn't stand up straight. I think I will have to bend it, so let's hope it doesn't break...
The "Tomato Growbag Cane Support" is widely available, but I got mine from The Gardeners Shop in Croydon, Surrey, simply because their postage rates were the cheapest I could find. I paid £8.94 for a pack of 3, including VAT and delivery.
It will be a long time before I put these new acquisitions into use, but when I do I will let you know how they perform.