That's not how it works. You can't just find a firmware for another device that uses the same (hopefully?) SoC and use it. The firmware is specific to the device, not the SoC.
Although its not completely impossible, I guess there's in general not much work being put into making OpenWRT work as operating system on a tablet device.
Is it possible to pick an arbitrary firmware with the same SoC? Certainly not.
Is it possible that someone made a specific firmware for your device? Possible, but unlikely.
But what I actually meant was: I doubt many people even bothered with creating an OpenWRT firmware for any tablet (not your specific ones) just because there's not much use for OpenWRT on a tablet. So it's not completely impossible someone ever gave it a try (who am I to judge what others spend their free time with doing) but I wouldn't even dream of doing this.
Could you please tell us what you're actually trying to do? No one ever just wanted to install opkg just for the sake of having installed opkg. You clearly want to run some kind of application, but you're not telling what you're trying to do. You only tell us about you not knowing how to make the very first step no a path you alone decided what stages you should go to without telling us the direction.