The general rule is that MODx (0.9.6.x - 1.0 Evolution; 2.0 Revolution is another matter altogether, but is still in beta) starts to slow down at around 5,000 documents. So if you don’t expect to have more providers than that, using a document for each provider, then using Ditto or the
A-Z listing snippet would be the quickest and easiest way. As netdoctor said, use TVs to add
more fields for the document.
Depending on what, exactly, providers are (are they going to be actual registered users of your site?) you can also use WebLoginPE in Register and Profile mode to gather data from them, Manager mode to have an admin user manage them, and Users mode to list them.