Stencils would be difficult to use with minis, for a few reasons.
First would be detail level. With repeated sprays, you're going to clog up your stencil. Points become rounds, if you catch my drift.
Second would be carving your stencil into a piece of plastic [?] that's small enough that it will fit on the small surfaces you wish to spray, but not so small that you wind up overspraying around it.
Third, would be that if your stencil isn't perfectly flat against your surface, you'll have "leaks" under your stencil, that will wreck the detail.
You could probably find some kind of printable transfer sheet, and then you could make your own transfers.
Might not be any cheaper, but you could.