You must definitely write: a practice, but to practise.
Exactly when as you write : an advice, to advise; or: a device, to devise...
The reason is etymological, from French (and don't ignore that two thirds of the English vocabulary comes from over The Channel, it explains numerous oddities in English spelling and pronunciation - in fact regularities in French, or rather old French).
Take for instance:
a) advice: the old French was "un advis" (the "d" is now dropped); it was pronounced, and still is in Southern France: avisse (avi in the North).
b) advise: the verb was "adviser" (the "d" is now dropped, too); it is pronounced avizer
The French rules ensue themselves from Latin.
in practise is therefore incorrect; the confusion comes from the USA; the immigrants have not always been good grammarians, and they still have no reference like Oxford or l'Académie