I'm in SW Colo and everyone hauls their water in my area. Pickup bed trailers are VERY common for this.
You have tons (pun?) or options. If you can't find anything but a half ton, then fine. Springs are EASILY added and I do this to ALL my trucks because a truck that sags w a load in the bed annoys me to no end. I added springs to my '05 Canyon, I added springs to my wife's '02 Silverado 1500, I added springs to my '95 F250 and I added springs to my '97 F250 Powerstroke. My '90 Bronco is leafs at all 4 corners and I've messed w those more than I care to think.
Do not worry about what the springs came from, leafs are leafs esp for a trailer. Ideally you should match width and most springs will be 2.5" or 3" wide (mostly 4wd Ford here). For a trailer you could probably put 2.5" springs in a 3" spring pack (but not the other way around).
Try to match arch but for a trailer again it's not that important.
Some center pins will be offset (Ford), some will be centered in the length of the spring (Chevy). No matter, it is perfectly permissible to cut springs (use an abrasive wheel) to whatever length you need. I wouldn't reco drilling spring steel, and for a booty trailer it's not worth the time. If one of the springs you add is a bit assymetrical front to rear, it's not a big deal for a trailer.
Get new u-bolts, any spring shop can do it. If you don't have a spring shop, look in the yellow pages. I order from Denver which is next day UPS even w standard ground.
You'll need a new center pin since your spring pack will be effectively getting thicker. If you can't buy locally (I never have) remove the existing center pin and figure out the diameter, then grab a Gr8 bolt of same dia and new length. If you don't have a lathe, chuck it up in a 1/2" drill, lock the trigger on and clamp the drill in a vise (not necessarily in this order LOL). Then take a grinder and grind the hex head down until it fits the hole in the spring perch on the axle. Some axles will not even require this and the hex head will fit fine as is.
This is the poor man's lathe LOL.
If this all sounds complicated, IT IS NOT. Leafs are extremely simple, and it will all make perfect sense once you get into it.
Have some c-clamps or Besseys on hand to clamp the pack together when you remove (and probably install) the spring pack center pin.
Now regarding actual axle strength, I think you'll be fine. People overload 1/2t trucks everyday. This doesn't mean it's a good idea, but there's a margin there. Plus your trailer will only be handling the weight, not dealing w the twisting forces of drive torque. If I had my choice I'd go w a Ford 9" only because it's not a c-clip axle, but otherwise a Ford 8.8, GM 10bolt or Chrysler 9.25 will be fine. Sometimes people remove the pinion, but then you need to cap the snout and do keep the axle topped off w gear oil.
I almost never add a single leaf given the work involved. I'd add two. If you get an identical donor pack, you can make a half-*** military wrap by taking the MAIN leaf and cutting the eyes so they form a half-round radius to cup the actual main leaf in your pack. This will give you the greatest increase in load capacity.
)
