The simplest thing would be to make one, by buying some flexible hazel rods, and sticking them in the soil, bending them over, and tying or weaving them together. It's a challenging little project.
If you want to buy one, then they are available commercially (possibly made of metal).
The concept seems to have been invented in 1984 by one Lady Tollemache of Helmington Hall:
"Lady Tollemarche’s new rose support, her brilliant idea, simple but effective, is the perfect way to have the best display from your old-fashioned roses. Use one and you will be amazed at how better you can see and smell these beautifully formed, highly scented varieties, whether as simple specimens or in your mixed borders. Much admired by fellow gardeners and visitors to the gardens, the Helmingham Rose Support system is now available in kit form with full assembly instructions."
I believe you can buy them (in metal), for £35, through the Helmington Hall Gardens website: The Helmington Rose Support
That page also has a slide show of images of roses growing over rose domes.
Given the new found popularity of the concept I'd be surprised if hazel ones don't find their way into garden centres before too long, though I don't know of any suppliers.
