I'm reasonably confident that your friend has got the name wrong.
Firstly, a Google search for 'zimmer crackers' doesn't find anything relevant.
Further, a look at the Intellectual Property Office's website finds 12 registrations for 'Zimmer' as a trademark but none of them relate to foods.
Spar is a 'symbol' group of retailers, which means that it's made up of independent retailers who join the Spar group to be able to advertise under their name and to take advantage of the bulk-buying power of a large group. (There are over 2600 independently-owned stores trading as 'Spar'). As such, while many of their products will be bought through the Spar network, they're also free to source products from elsewhere. So it's not possible to know whether other Spar stores might sell them. However you could try searching for a local member of the Spar group here:
http://www.spar.co.uk/StoreFinder/
Chris