Here you go, this is the deal here:
http://www.ebay.com/itm/4-Boat-Trail...-/260899109522
(4) springs for under $100. They are 20" 3 leaf 1750lbs rated.
Your axels are only 3500 lbs axels so going with heaver springs than 1750 would stiffen it up but you are still limited by the rest of the axel with 5 lug hubs and smaller straight axel spindles. Most 3500 lbs axels are straight spindles with two 1 1/16" or two 1" bearings. The 5000 lbs axels will have the tapered spindles with a smaller 1 1/16" outer and a 1 3/16" inner plus the hubs will be 6 lug also.