I have a small 3 insert face mill that takes 1135 inserts. They measure approx. 6 mm wide x 11 mm high x 3.5 mm thick with a 2.5 mm screw. I get them from Aliexpress for approx. $20 for ten. I have an X2 mini mill and I find that the aluminium cutting inserts give me a very good finish in steel compared to the steel specific inserts as I can only take light cuts with this mill. The ones I bought were Mitsubishi "branded" but perform very well on my small, bendy, loose, toy. Have chipped a few in harder metal but you can change to the steel specific inserts (cheaper). Hope this helps,
John.