Depends on your requirements, certainly you can find resonably priced heavy grade products from farm and industry. Anything over 40-50 gallons is not going to be cheap though. I like fifty gallon drums, free, but they may not work for your rig.
Smaller (27 gal) are easy to find in rigid plastic and cheap ($15.00) http://www.lowes.com/pd_44066-51438...tId=3033542&pl=1&selectedLocalStoreBeanArray=[com.lowes.commerce.storelocator.beans.LocatorStoreBean%4030823f71]¤tURL=%2Fpl__0__s%3FNtt%3Dplastic%2Bcontainer&storeNumber=2530&Ntt=plastic+container&langId=-1&errorURL=UserAccountView&storeId=10151. Depends on how often you want to top off and/or if you could use multiple reservoirs instead of one large one.
They work well under a 3X3 tray and can be left for over a week, in my experience, without going dry should the need arise.