First you need one more thing - a reason for the ROM. Do you want a ROM that runs faster? One with more space for apps? One with some particular function not in the stock ROM? What functions aren't important to you (in case the particular custom ROM doesn't include them)?
Flashing a custom ROM just to have a custom ROM is like cutting your finger off just to see what's inside it - the pain isn't worth the gain. The first thing that happens when you flash a ROM is that the warranty is void. The second is that you risk turning the tablet into a plastic box full of parts that don't do anything.
And the final problem you face is that there usually arn't any custom ROMs for those cheap Chinese, "make a thousand, then go into the artificial flower business" Android devices. There aren't enough of any single model to bother spending the time and effort on. Most people don't flash custom ROMs, so if a ROM developer spends 6 months developing a ROM for a $150, 3 generation old tablet, and one or two people install it, he wasted $200 and 6 months. Don't expect to find custom ROMs for anything made in Shenzhen, not even "Samsung Note 4" phones (which aren't Samsung, and are somewhere around Note 1 technology, but not as good, running a non-standard version of Android).