I ran into the same thing over in Alabama a few weeks ago. I went to the Lowe's and they had just about everyting but the exact same part. The home owner suggested Marvin's (a local store) and they not only had the part, but much better prices on many things than Lowes. In fact, while I was there I ran across a deal they had on DeWalt 21 piece quick change bit sets for $2.95 ($18 at Lowes/HD)... I bought 5!
I think different Lowes have different inventories. A couple of the Lowes I frequent recently stopped carrying 3/4" foamboard, but the one a few miles away still stocks it.