Assuming it's a fairly recent production Salient then any PLCE compatible side pouch will fit.
Green PLCE side pouches will fit and they have better zips IMHO than the Lowe version but you loose out on having shoulder straps on the pouch and have to use a yoke.
Sapper 1 is correct the Arktis side pouches will also fit as will karrimor, and webtex and medics pouches etc etc....