Hi Mikkel
Karlik is a cross from former USSR between some sorts, it isn't clear, but probably some citrus unhiu, citrus leiocarpa, citrus paradisi, poncirus trifoliata or fortunella.
You can buy this sort in nursery Adavo, in the city Velky Osek - it is probably 30 km east from Prague.
They have web page
http://www.rakytnik.com/Eshop/index.php?main_page=product_info&cPath=1_3_15&products_id=41This sort is 25-40 cm high, 2-3 years old ane price 192 CZK - it si less than 7 €. I don't know if they send trees to other countries
Michal