Shouldn't be too difficult to create, just will take a few generations to get started. Both mottling and lavender are simple recessive genes; the regular Mottled is just a black base bird with mottling added, and the Lavender is a black base with Lavender added. A simple cross of Lavender X Mottled will give 100% Black offspring, all of which will be split for (carry) both the mottling and lavender recessives. I'd recommend breeding the offspring of this cross to each other for the quickest way to get Mottled Lavenders; if you do a sibling cross (or non-sibling if you keep two breeding pairs of Lavender and Mottled), then 25% of the offspring of this generation will be Lavender Mottled. 50% will be split for mottling and/or lavender genes and can be used to breed back to their parents/siblings/other splits, and 25% will be simply Black with no recessives.
The chicken calculator
might be of interest to you, it's very helpful, especially when working with simple genes like this. Does all the math so you don't have to.