We present a search for a narrow resonance in the tbbar mass spectrum using 1.9 fb-1 of ppbar collisions at sqrt(s)=1.96 TeV recorded with the CDF II detector at the Fermilab Tevatron. We select events with a lepton, neutrino candidate, and two or three jets from which to construct the tbbar mass. We quantify the result using the model of a massive Standard Model-like charged-boson (W') decaying to tbbar, but we are generally sensitive to the presence of any narrow state decaying to the third generation. For a purely right-handed W' with Standard Model couplings, we set a new limit at 95% confidence of sigma x BR(tbbar) < 0.28 pb and M_{W'_R} > 800 GeV/c2. The limit increases to M_{W'_R} > 825 GeV/c2 if decay to right-handed neutrinos is forbidden. For a right-handed W' (or left-handed W' with negligible interference) with arbitrary coupling strength, we set limits on that coupling as a function of $W'$ mass between 300 to 950 GeV/c2.