Beeker1 coins has recently obtained some of the more difficult to find Elizabeth I sixpences. Any of the dates around the Spanish Armada utilizing the Crescent Mintmark are difficult to find. Beeker1's site has an example of all three dates 1587, 1588 and 1589. Of course 1588 is the year the English defeated the Spanish Armada and is the most popular and most difficult to find of the three.
Also any of the dates in the late 1590s are hard to find. Currently Beeker1 carries a 1595, 1598 and 1599 sixpence. When was the last time you have run across a 1599 Sixpence? There is also a very nice example from the last year of Elizabeth's reign a 1602 Sixpence. You can find these at english-hammered-coins.com.