Nice trailer
The tubs are called IBC's (Intermediate Bulk Containers) .... They come in very handy. You can usually get these for around $25 used. For watering applications, I would recommend that you purchase ones that were used to transport food or food ingredients. You can search the web for local places to get them or check for local places on eBay. The only issue is that the threads are not typical on the spout (metric maybe?)
If you cut the top off, they also make great firewood carryalls
They are very handy to have laying around