Not sure what your budget is or how much deck clearance you have... but there are a lot of transfer type tanks shown on Northern Tool. You can also find them stocked in Farm Supply stores.
I have both aluminum and steel transfer tanks on my generator trailers. Aluminum is more expensive but is worth it in the long run in my opinion. Aluminum won't rust inside or out like steel eventually does, especially with storing any biodiesel (hydroscopic) blends.
Here is a
Northern Tool link to take a look at several offerings with some 36 & 45 gal versions. I have one of the 36 gal "Better Built" tanks and its OK but it started to rust inside after 2 years. It also had a lot of weld slag inside it that I had to clean out before using.
BTW, my best find was a used aluminum
50 gallon refer tank project (the ones seen hanging from the belly of 18 wheeler semi refrigeration trucks) with cradle that I found on Craigslist & purchased for $60 & then "rejuvenated" it.