Genuine interstellar grains are found in meteorites (Clayton & Nittler 2003), but, because present search techniques rely on isotopic anomalies, the grains that are found are limited to those formed in stellar winds or ejecta. Theoretical studies of grain destruction by supernova-driven blastwaves lead to estimated grain lifetimes of only ~ 2 - 3 × 108 yr (Draine & Salpeter 1979; Jones et al. 1994); this, together with the large depletions typically seen for elements like Si implies, that the bulk of interstellar grain material must be regrown in the interstellar medium (Draine & Salpeter 1979, Draine 1990).
Because the typical interstellar grains don't bear a distinctive isotopic signature, we don't know how to identify them in meteorites. We are therefore forced to study interstellar grains remotely.