Re: Is this allowed?
Well,There is currently nothing in the rules about weapons being restricted to certain factions. The general rule of a thumb is that if you are in a faction that has "restricted" weapons such as Suppressed Pistols and Knifes. Then you are trusted to not abuse them. The general reason these weapons aren't readily available is because not every player can prove to be trusted with things such as suppressed weapons.
I guess for the sense of realism, A hardware store wouldn't be able to create homemade suppressors that were anywhere near as nice as military and law enforcement grade ones. My best recommendation would be to create a long burdening RP for the weapon. For example, RPly talk to someone who works at Marvin's hardware then maybe go talk to another guy, Then get something for that last guy and create a little quest, if you will, for obtaining the suppressor.
I myself am not entirely sure. Do not take my word on anything I just said, For now, I will tag this for an admin and see if we can get an exact answer from an administrator as they are hopefully much more knowledgeable about something like this than me.