springs
A google search of spring repair shops in your area, or check with truck repair shops.
I lived in the Detroit area long ago and I used to take my springs to a place called "Red's Springs". On Telegraph in Dearborn Heights. They would custom build a set of springs to whatever specs you wanted. It helps to know the spring load rate, (found in the spec section of your manual).
$150.00 sounds like the going rate these days, including bolts and bushings.