Kuchnie Swiata has it :) You should be able to order it from them - I'm not sure if they have an online shop, but they're in some cities - and if not, they should be able to sort you out with some supplies on a wholesale basis :) It's 5zl a can in the shops.
If I remember, you're in Olsztyn - if so, you're in luck! :) Their wholesaler is there too - so you should be able to order a decent amount at a good price :)
10 - 540 Olsztyn
ul. Dąbrowszczaków 13/2
tel.: 89 535 07 23
fax: 89 521 03 94
pon.-pt.: 800 - 1600
sob.: 900 - 1200
Even if they don't have it in the store, they can order it easily for you.
Whats more important is....where can you get dandelion and burdock from...?
M&S sells it here, I think? Shall investigate for you...