I am looking for a site that I can input my BM color selections and will give me a similar color from either SW or Porter.

That's not the way it's done. It's far more simple. All you have to do is supply the brand, color name and color number and the paint counter -whatever paint counter- can mix that color for you. That's their job.

This is a good fit for you because as you say "they don't need to be an exact match, just close." Cross-color-matching brands works. People are successful doing this every day. Just have to be clear on what to expect. Some 'matches' will be spot-on but there's no guarantee as some 'matches' will simply be "close".

It's important to decide what you can live with as far as color 'matching' goes. As I said, you're all set because you've already resigned yourself to close enough is good enough.