There will be incandescent lamps available, especially for historic fixtures; although you may have to smuggle them in from Canada ... like I do with my medications. Your client and others want the type of light they grew up with and are used to. But even incandescent lamps were a pretty new technology when Tiffany lamps were being made. I can imagine the people of the period saying that they would hang onto gaslight and not switch over to Edison’s newfangled bulb. People love incandescent light for the color, the dimmability and the initial low bulb cost. Consider CCFL (cold cathode fluorescent) lamps instead. A-lamps, flame-tip lamps and G-lamps from companies like Literonics (pictured) look like their incandescent counterparts. They dim with an incandescent dimmer and have a lamp life of 18,000 to 25,000 hours, depending on the type. Although they cost around $12, they do save around $68 over the life of the lamp (based on $0.11/kwh). Hopefully, in the future, mercury-free ESL (electron stimulated luminescence) lamps will offer most of everything we love about incandescent, while providing long life and energy savings.