Those big buckets that people use for horses or washing baskets :) they love it!
I washed our son in a wide bucket (don't put too much water in or it'll be too heavy to carry). Holding them in the shower also works but I found he was very slippery so had to have a good hold of him!
Sink if it's big enough. Large washing up bowl. Or I've also had a small blow up paddling pool in the bottom of the shower. Actually showering is fine too but if it's part of the bedtime routine then it can be too stimulating x