Anything big enough to hold the water will work. You might have something laying around. I use cheap tin foil pans. You could get something fancy if you want it to look cool. A hard pan would be nice so it doesn't try to bend when you pick it up.

I'm just going to get some oil drain pans I think. I've decided I'm going to go with 2 plants to start out. The smart pots I got are 12" at the bottom and the oil pans are all I could really find that would leave enough room and shouldn't overflow. I'm actually thinking I'm going to use a small shop vac to suck water out instead of trying to move plants and drain the pans.

Note to self though, don't order soil from amazon. My first package was destroyed in transit so they sent another, and I just got another email saying that shipment is delayed. Still don't have my seeds so it's not a huge deal but dayyum.