A splicer costume is fairly easy to make. Some nice clothes, covered in blood, a masquerade mask, and a weapon.
You can grab some nice cheap clothes from your local Salvation Army/Goodwill/thrift store.
Masks, you can either make it yourself or buy one from some one who does (*shameless plug* Me
The weapons are mostly wrenches or pipes if you're going for a thuggish splicer look. Other ones, find a gun or some hooks.
Hoped that helped Buhdog11