I got mine at the beginning of April of last year, but went back in mid-May (the weekend before finals) as one last hurrah before graduating. In May, I couldn't find any of the ones that I had bought in April (trad. ears in blue with a tiny mortar board on top and "Class of 2008" on the front, I got it because I wanted my name on the back), but they did still have plenty full-size mortarboards with furry mouse ears attached. Mine were about $15 and the others were wither $20 or $25 (can't remember). They did have some furry ears that lit up. I would think that they'd continue to produce them well into mid-June because of graduation dates. If all else fails, you can still get one of the new and nifty "Just Graduated" buttons that they're handing out as part of the WWYC campaign (wish they had 'em last year).
A picture of the type of ears I got:
A picture of the other ears available:
http://www.mouseshoppe.com/Merchant5/product_images/11783251S.jpg
A picture of all the pins one can get at City Hall (including the graduation one):