If each axle has exactly 2,500 pounds on it, and 500 pounds more is on the hitch, then the total weight will be around 5,500 pounds.
Your axles are WAY under rated IMHO. It relys on the exact weight to be on each axle and each tire. If you actually checked each tire weight my guess is if the total weight is over 5,500 pounds, then at least 1 tire will be over the rated weight.
My suggestion? Well with only 5,500 pounds, then disk brake upgrade is probably overkill. But if the GVWR was in the 8,000+ pound range, I would suggest going with disk brake conversion, or at least consider the costs. If the upgrade is within $1,000, I would much rather have the disk brakes and effective stopping power. But again, your trailer is only 5,500 pounds.
ETrailer.com has the parts, just look at their website, and call if you have questions, or don't list everything on the website, but they can get it anyway.
They also sell mounted tires on steel rims, for a fairly decent price. But beware, that shipping fees can eat up any savings. If you are having axles shipped via a big truck on a pallet, then adding a few rims and tire combo's will not significantly change the shipping fee. 300 pounds and 450 pounds are very close in price, usually liftgate service adds an additional $35 fee (at least it did for my air conditioner delivered from Florida that was 350 pounds all together).
I was able to find a RV supply store near my uncle's place in Virginia that had steel rims with bias ply tires like the ones on my tow dolly, and I was able to get a spare tire for only $53. It was 14" and rated at about 1,600 pounds.
Good luck,
Fred.
