We demonstrate a new tunable mid-IR laser source based on the guided-wave frequency conversion of two diode lasers operating in the near-infrared. Important features of this laser source include portability, room-temperature operation, freedom from thermal cycling, smooth tunability, and modular construction using readily available components. The source, when fully optimized, will maintain the properties which have made lead-salt lasers so useful for atmospheric trace gas detection, including sub-Doppler linewidths, microwatt-level output powers, and amenability to detection techniques based on frequency modulation. We describe the design and construction of the laser source, including its key component: a waveguide fabricated in periodically poled lithium niobate. In addition, we present a laboratory absorption spectrum which illustrates the potential usefulness of this laser source for the detection of atmospheric methane. Difficulties encountered when making the transition from a laboratory tabletop device to a portable device are discussed.