Changing immediately from TN to B requires an I-539, on which you request up to 6 months to "wrap-up" US affairs and head out of the country. It must be submitted on the last day of TN status (there may have been some new leeway introduced in the omnibus changes announced in January that allow you some time to file a legal change after ending TN).
After submitting, you are in B2-pending status, which is in effect until only you leave the country, which should still be your main aim in this. You should not be waiting to hear back from CIS on your petition. If you do find another TN, you would go to border (or file I-129). You cannot leave US and re-enter on that B2, you would have to qualify for B2 like any other tourist (well-documented foreign residence, tourist purpose for entry, definite plans to leave), so when you leave US the first time after filing B2, be prepared not to come back without an iron-clad TN or proof of a new life abroad (DL, lease, utilities, etc).