  We report the results of a search for long-period ($100<P<600$ days) periodic
variability in SDSS Stripe 82 standards catalog. The SDSS coverage of Stripe 82
enables such a search because there are on average 20 observations per band in
$ugriz$ bands for about 1 million sources, collected over about 6 years, with a
faint limit of $r\sim22$ mag and precisely calibrated 1-2% photometry. We
calculated the periods of candidate variable sources in this sample using the
Lomb-Scargle periodogram and considered the three highest periodogram peaks in
each of the $gri$ filters as relevant. Only those sources with $gri$ periods
consistent within 0.1% were later studied. We use the Kuiper statistic to
ensure uniform distribution of data points in phased light curves. We present 5
sources with the spectra consistent with quasar spectra and plausible periodic
variability. This SDSS-based search bodes well for future sensitive large-area
surveys, such as the Rubin Observatory Legacy Survey of Space and Time, which,
due to its larger sky coverage (about a factor of 60) and improved sensitivity
($\sim2$ mag), will be more powerful for finding such sources.
