You're right, soda & crystals are the same thing, but since I've found mine under the name of soda crystals it may help you to do your searches under this name.
Dri Pack do my crystals (www.dri-pack.co.uk
), and I think if you go on their site there is the possibility of buying online (or finding retailers in your area). They also sell Borax.
Otherwise, I know in my online searches I looked for "sodium caronate decahydrate" and found some (ie: the pool cleaning suppliers, etc).
There are not any Asdas in the Republic (the luck of living 5mins from the border
) but crystals can be found here with enough persistance & googling. Good luck!