For now, I have a reasonable supply of R39 30W reflector bulbs to keep my 3 (working) lava lamps happy, although they come at a premium and are hard to find.
I tried to look for possible alternatives, and saw that e.g. 25 W golf ball bulbs fit in nicely under the large-ish liter-class globes and seem to work too, although they mostly produce large, slow blobs under cold weather.
I was thinking about the possibility of retrofitting the base with a 12V halogen reflector lamp, using the transformer of an old desktop halogen lamp. Compared to golf ball bulbs, all of the heat would be directed upwards effortlessly, and durability would also be much greater than mains incandescents. They are also cheaper and more readily available than specialized R39 reflector bulbs, and come in 25W and 50W ratings.
Of course, "heat shielding" models cannot be used, as those tend to trap most of the IR radiation inside the bulb itself and it would not heat the globe properly...also, I don't know if removing the UV-filtering glass that most of these lamps have would have an adverse effect on the wax, on the long term.
Has anybody tried this?