Robyn I've Googled and you can buy seeds but doubt you'd get a good plant or anything like you've seen in Spain, unless you grow it in a conservatory. I might be wrong but I don't think it is suited to UK climate.
Even in Greece people buy them as small plants from nurseries or take cuttings.
http://www.jparkers.co.uk/bougainvillea-lilac-0001294c not seeds but little plants to be nurtered on.
Apologies Robyn the tropical site that I found in UK doesn't sell seeds but did appear under seeds
It seems that they seldom set seed some have in S. America and have been harvested by bag method and propagated. Maybe read up about taking cuttings seems you can do that, and you have a source Just out of interest roughly where are you located?
I hope you have success, they are beautiful plants but would not survive in Yorkshire.
You never know someone on the forum might grow them and they have produced seeds
Saying all the above I assume you mean a plant that will scale a wall and pergola