One of the way how to get mice is to find sims social fairy dust, but where to find this item ? take a look for fairy dust picture here :
well it’s just like the new snails, which you can find it pop up on your yard the bad thing is.. it’s not always appear on your garden because it’s random.
now click fairy dust and investigate it !
if you lucky then you’ll get mice drop with some simoleons and xp, and yeah the mice won’t always drop though the chance is small
so good luck finding this item because you will need a lot of mice for the great dragon tower