RIA makes a wide range of 1911s. From GI up to TAC and double stack models. Knowing which one you specifically have will help find a holster.
If it's a 5" barreled RIA 1911, with no rail, it is simply the equivalent of a Colt Government Model.
Most holsters are made using a "Blue Gun" ( http://www.bluegunstore.com/colt1911.aspx ). Nobody will make a holster specifically for a RIA 1911. It's just a 1911. SIG is about the only 1911 that you have to specify maker, since some models have a SIG-unique slide. There may be some oddity to an RIA 1911, but it isn't significant enough for a holster to matter.
5" Barrel No RailBarrel length is significant, as is whether it has a rail or not, but other than that, almost all 1911's are the same (SIG being the rare exception).
Any holster listed for a 5" 1911 or a Colt Government Model will fit your RIA.5" Barrel No Rail
Pick Colt 1911 5"5" Barrel No Rail
but for instance BCA dose not offer a generic option it literally asks you to pick from a list of specific models