As stellar black holes (BHs) interact with one another in globular clusters, BH binaries form, inspiral, and merge via gravitational wave emission. These dynamically formed BH binaries may account for a fraction of the sources observed by LIGO/Virgo and, earlier in their inspirals, may be detected at millihertz frequencies by LISA in nearby galaxies. M87, known to contain over 104 globular clusters, is a promising candidate for such millihertz sources. Utilizing N-body cluster simulations, we investigate the binary BH population in M87 clusters. Although thousands of sources in LISA's frequency range (10-5-1 Hz) are expected in M87's clusters, we predict none are resolvable with sufficiently high signal-to-noise ratios. We predict the nearest globular clusters in the Milky Way and clusters found in distant galaxies (d ≳ 100 Mpc) near LISA's detection horizon for stellar BH binaries are more promising hosts of resolvable LISA sources.