2002 Toyota RAV4 Catalytic Converter- PLEASE HELP!

Hello, My car recently got the catalytic converter stolen off of it over night. Now, you guest it, it's really loud and it's driving me crazy! The mechanic is telling me that the part is costly, so I was wondering about purchasing a universal cat converter. So my question to you is, Does it really matter what universal converter I choose? And if so, Which ones are the best for the lowest price? Is there a particular brand that I need to go with or stay away from or does it even matter? Do I need to pay attention to the series, sku or descriptions of the parts? PLEASE HELP. Any advice you give will be helpful. One mechanic is telling me that I needed a specific type of universal converter because of where the cat needs to sit so that it would not have my check engine light on all the time and not fail the annual GA emissions test, but the part costs that he's quoting me is still costly compared to another quote i'm getting from a different mechanic. So now i'm undecided on who to trust? I really don't have the money to make the repair, so i'm really looking for the cheapest way to fix the problem. I just want the noise to go way, and to be able to pass the annual emission's test. I don't need any unnecessary costs. Please Help!