This sounds scammy. The joint sponsor is supposed to ensure that the immigrant will not become a public burden. If they ask 'how do you know this sponsor', and you reply 'Well, my lawyer found a sponsor who I don't know, and I paid them $500 to be a joint sponsor', that won't look good, IMO, and won't create the sense of comfort needed to assure them that WE (taxpayers) won't end up having to pay for her.
I think it's worth you making sure that you have the financial capability to support her. Take your time figuring out your income, expenses, future earnings etc. And then sponsor her with confidence.