At the risk of answering a nearly closed question and related to your willingness to consider creating your own, it would appear that casting is going to be the only practical solution available, unless you have a tremendous budget.
I did a quick search using The Google for the terms "18 inch acrylic cylinder" and found a number of suppliers. You'd have to do the math for length vs. volume vs. diameter to know the exact or approximate size. One of the vendors lists 0.250 inch wall, 20 inch diameter, US$583.98 per linear foot!
On the casting side, which does fall into the crafts aspect, you might consider to purchase a five gallon bucket and use that as the beginning of a mold. If you did not require exactly five gallons, you would pour the casting acrylic liquid into the bottom of the bucket. When the bottom inch has cured, construct an inner mold sleeve, sealing it well to the bottom and fill the outer ring with casting acrylic liquid.
If you go that route, use caution regarding the volume of acrylic you pour, as it can generate enough heat to melt the plastic bucket.
Alternatively and much more expensive is to create a silicone mold to then use to pour the acrylic. You can find silicone mold products which can handle temperatures high enough to cast metal, certainly capable of handing the heat from acrylic reactions.
This would be a project of some labor, some experimentation and possibly some undesirable results, typically called learning, but is probably going to be less expensive than buying six hundred dollars a foot plastic tube and still having to seal the ends!
I did a bit more research and discovered that a five gallon cylinder will be 12 inches tall and 11 inches inside diameter. Those numbers confuse me, as a five gallon Home Depot bucket is larger than a foot across and a foot tall. One gallon is 231 cubic inches. Five gallons is 1155 cubic inches. Again with The Google, the formula says 11 inches diameter by 12 inches tall is 1140 cubic inches. That's close enough. Maybe the five gallon buckets are more than five gallons or maybe I'm calling them that and they aren't.
At any rate, 12 inch inside diameter, quarter inch wall tubing from San Diego Plastics (the reference I used) is "only" $182.74, far less than the previous figure. Shipping might not be too bad for a foot-square box either.
Shucks, I preferred the casting method for the learning experience, but it isn't going to be cheaper than the retail purchase.
