Check the eBay. To be exact, last time I bought one for $12, but when checked yesterday - there were $6-7 offers. Just ordered one for $5.95 to verify if that's not a fake. The $12 one is class 10. However, most of the data on those emergency cards will be static (documents, photos, home videos, books, music, games). So I would not be concerned much even if that's in fact Class 4. Actually the older slow cards should be a bit more reliable (larger components). Even better to use old 2Gb cards, as they are made by SLC technology (not MLC as modern inexpensive high capacity cards), which data retention time is 10 years (vs 3-4 for MLC).

By the way, keep in mind, that any solid state flash cards/drives (SSD) cannot be stored forever as DVD disks (the later also depend on storage environment a lot, though). It is crucial to have them powered at least twice a year, and rewrite with the data every 2 years, otherwise the stored data may start deteriorating. Also, it is better to use fresh (brand new) cards for long term storage, as with every rewrite cycle the data retention time will be reduced. In that regard ordinary hard drives might look more attractive as they seem to keep data much longer, however, for hard drives, there are three main factors affecting their data retention time: magnetic field breakdown, bad environmental conditions, and mechanical failures. So, it is recommended to rewrite long term storage hard drives (which are sitting on the shelf, not powered) every 2 years as well, including a mandatory low level reformatting procedure.