I use random string generators with notepad++ regular expressions, which is kinda stupid way to find numerics. I'm not good with programming, in fact I forgot how to program 8-9 years ago What do you use? Do you write custom scripts?
Last year I had built a desktop software in C++ to generate domain names, now it came in handy.
It has groups of characters like Numbers, Alpha, Most Frequent Letters, Western Premium, Chinese Premium, Chinese Numbers (No 0 or 4), Chinese Best Numbers (2,3,6,8,9)....infact I added these groups just lately
I have used it to find some good pronounceable 5L and some numerics. I still have to input those domains into Godaddy bulk search to check availability.
There are available domain generator in free version http://whic.ru/en/pro/price
You can generate by masks, by lists etc
Also bulk whois checking available