With the type of engines the average consumer has, it isn't really necessary. If the engine is run regularly and the oil changed on an appropriate schedule (using the appropriate modern oil formulation) there will be enough entrained oil present to ensure proper lubrication on start-up. Larger engines (like cars) are designed with oil galleries and hatching to hold oil for startup. Small engines often don't even have oil pumps, instead relying on splash oiling.

Where you see pre-oilers is more in large industrial engines, especially engines that see infrequent use (like in emergency back-up generators). Why? A couple reasons. First, these engines are so physically large that it takes a while for the oil to reach all the necessary surfaces. Second, these engines tend to sit for long periods of time, allowing the oil to drain away from said necessary surfaces. Finally, these engines are often turbo diesels, which are more susceptible to things like oil coking and fuel dilution of oil. With that in mind, they often use a pre-oiler in conjunction with an oil bypass (filtering) system and possibly even an oil heater. This is done more to ensure oil quality and viscosity than anything else.

You also see pre-oilers in airplane engines, specifically radial engines, which apparently have problems related to oil pooling (though I have limited experience with airplane engines).

Anyway, If you're still interested in it, there are plenty of companies selling pre-oiling or oil accumulator systems on the net. I know amsoil used to sell one, but when I looked it up it was apparently discontinued. Just be sure to look into how they operate, because many of them require a separate pressurized air source.