Very tough to find, especially in high grade. These show up occasionally on eBay UK and other auctions sites and you should grab them while you can! A G/VG copy of the 1970 edition sold on eBay UK way back in Feb 2008 for £28! A nice mid grade VG/FN 1969 copy fetched £70 in Jan 2011.
Unlike annuals, Summer and Holiday Special came out in the middle of the year (there are occasional Spring and Winter Specials, not many). With annuals a copyright year 1968 would be intended for the Christmas market and beyond with that annual being the 1969 edition if it wasn't otherwise dated on the cover.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.