There is no true equivilant of an OTG connector for the iPad.
On Android you are only converting to one USB connector type to another. This is simple, and just about anything that can connect to USB (and you can find an app for) will work.
On iPads you convert from Lighting to USB, and while the Lighting cable is USB on one end, not all USB periferials are supported. To get most USB periferials to work with an iOS device it pretty much as to be made for iOS, and you need an app for that perifrial. The few exceptions are some card readers (mostly very low power ones and only for photo/video import), keyboards, microphones, and a few others I’m probably not remembering.
In short, there is no cable you can buy that will make random USB stuff work with the iPad.
So, what you really need to do is specify what you need to connect to the iPad, or what you are trying to do with the iPad. Once you do that members may be able to recomend solutions they use.
Note: There are some things that are extrememly difficult (or impossible) to connect to an iPad, which is why I own a couple of cheap Android tablets dedicated to my SDR (Software Defined Radio) hobby.