I went to a Northern Tool and found a pond pump, and use it to both stir RODI
water when I add minerals to it (just drop it in the bucket), and then to pump it up to the display tank.
Only problem is the nice small $10 ones often have a low head pressure, i.e. they won't pump more than 4-6' high. Check that first. But they have cheap ones. Similar on Amazon, etc. Look for "pond pump".
Here's the one I used: here
On a larger tank that I needed about 8' of head and more volume I actually used an old boat bilge pump. It's 12V so you need an adapter (though if you have DIY LED
's you may already have one), but the are waterproof, designed to not be damaged by running dry (at least briefly), have a decent but not huge flow rate, and often you can find them used cheap/free.