Where to buy an SR Latch
RS latches have several parts in the industry-standard 7400 and CD4000 series, all of which are quad packages in old logic families:


74LS279
CD4043
CD4044


Another alternative, with two parts in a package rather than 4, but more available in modern logic families (read: lower-voltages) would be to use a 74xx74 dual D flip-flop as an R-S latch by grounding its data and clock inputs, and using the PRE and CLR inputs as S and R inputs, respectively. In this case, I'd start with a 74HC74.