We present a large sample of 501 radio-selected BL Lac candidates from a combination of Sloan Digital Sky Survey (SDSS) Data Release 5 optical spectroscopy and the Faint Images of the Radio Sky at 20cm (FIRST) radio survey; this is one of the largest BL Lac samples yet assembled, and each object emerges with homogeneous data coverage. Each candidate is detected in the radio from FIRST and confirmed in SDSS optical spectroscopy to have (1) no emission feature with measured rest-equivalent width larger than 5{AA} and (2) no measured CaII H/K depression larger than 40%. We subdivide our sample into 426 higher-confidence candidates and 75 lower-confidence candidates.