In the UK, I often go to diy store gc as they do have
Phyllostachys Nigre, but not Phyllostachys aureosulcata 'Spectabilis'. Two or three weeks ago, they were clearing them for the Ccchristmas trees, so they reduced them half price. I got one for under £13. I split that into 6 plants.
If you want a phyllostachys Nigre, it has to be tall and mature as quite a few bamboos have black stems when young.

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I think you can buy Spectabilis online, but must say to the seller that you need to see THE GREEN STRIPE in the delivered product.
Or go for reputable companies and keep an eye out for their reduction. If not reduced, they can be very expensive. £40 for a 13L .. I was very lucky to have got mine for under £13!