Where is the oil filter housing located on the engine

Is it necessary to actually replace the housing itself or is just the gasket
what is the average mechanic cost to replace the oil housing gasket( I have the new part)
I have been given various diagnosis of a my cars condition small oil leak after I had my oil changed. I initially went back to where I had the oil changed and they checked everything and said that there was no oil leak suggested it may have been do to some oil spillage. After the leakage still persisted I went to several mechanics and got various diagnosis of the problem. The actually oil housing itself, others said it is possible it is the oil housing gasket that is leaking. I need your help please. If it is just the oil housing gasket what is the average mechanics charge to replace gasket if I have the part