Tomato supports

I’ve seen them for sale from time to time. Never used them.
I do something similar in the tunnel with string. Tied to a bar of the tunnel. Other end with a knotted loop.
That the tomato gets planted on top of. So it’s own roots hold it in place. Then just twist the tomato round the string as it growing. Obviously that’s wouldn’t work outside.

I used to use string, tying the bottom end of the string to a heavy metal washer or a stone to weight it down while I filled the pot with compost and planted the tomato. 
I reused these strings for a couple of years but they string had perished this year so they had to be replaced and I've used canes in the pot attached to another cane which follows the roof line to the apex. Because I use open bottomed pots I'll be able to lift them up around the cane to remove the old compost without disturbing the canes. That's the plan anyway.

I reuse the same string year after year as well. But I don’t use gardening string. I’ve got a few different ones. The starter pull cord works very well. I’ve also used electric fencing cord. That’s a little tricky to tie. ( this one won’t surprise you) builders string line. All of which last much longer than garden string.