B&H has Ilford MGIVFB 12x16 glossy or matt finish in stock but order by 1pm today as they go on holiday until the 23rd after that.

12x16 seems to be a fairly rare size compared to even 16x20 and 20x24. You could cut 12x16 from 16x20 (would give you a pair of 8x10 sheets as well).