There are plenty of custard recipes but all are based on commercial custard powder. What is actually the custard powder? Is it possible to make custard from scratch at home (without custard powder)?
Custard is a mix of egg yolk and dairy (often milk, but sometimes including cream as well), which is then heated. The ratio of yolk to dairy depends on the texture desired (with a high enough portion yolk, it will set; with less, it'll just thicken). The temperature its cooked to varies, but is usually between 70°C–80°C.
Custard powder is, as far as I can find, a mix of cornstarch and other thickeners, vanilla, and yellow food coloring. (You could read the ingredient label on yours to determine exactly what's in it.) It intended to imitate the above, but be more convenient (and cheaper). You could use cornstarch and vanilla (and maybe some other flavorings) to imitate it.
Depending on the recipe, I'd suggest looking for the version made with actual milk and eggs, or at least look for something similar to your recipe, to get the milk/yolk ratio and temperature from.