Your probably a bit late since the fruit normally ripens in the next few months. But early next year the fruit goes on sale. At least it does in Ireland and England where people really like it for home-made marmalade and you can order it very cheaply and the fruit is meant to be packed full of seeds. And as an added bonus you can make some marmalade.
You can definitely order them online but since it might not be that easy for you and since the fruits are freely available to most people in southern Europe then it might just be better to wait a few months and ask again
In England lots of people like to make homemade marmalade so in the season they can be found easily enough if your looking and lots of websites say that they sell them in the season. I even saw lots of ads on amazon UK all out of stock of course. I did see a Spanish website that ships them through out Europe but smallest order was 15kg. I know they are for sale in Ireland but I'm waiting for the season before I look for them. I was just planning on ordering them from the UK but I'm not too sure if that will work since they are meant to leave the EU soon.
If anyone want too, there are 24 seeds and I can get more, you can ask for it before I go to the postal office.
I know I'm a bit late, but if you have any spare citrus aurantium seeds, I'm always interested!
I think that there are some oranges in the trees, but I don't know if seeds are viable now. Next day that I pass near, I'll take one to check, but if you are not in a hurry I'd better send you some in February. Remind me.
Bitter orange is used a lot for jams, here the most sold are strawberry, peach and bitter orange.