Yup. Krylon matte sealer.
I bought my matte sealer from Michael's (a nation-wide chain of craft stores). It's not that easy to find anywhere else...
Be _sure_ to use the matte, as the satin sealer is much glossier than you might expect. The satin is more durable, though, so I usually apply a couple of coats of satin for durability, then finish it off with a couple of light coats of matte. The result is pleasingly durable.