Hello and welcome to Just Answer. I will try and assist you with your question.
It seems that this is an issue that more than one person has had. From what I am finding is that most likely the springs are Skyjacker. From what I can find these (LINK) are supposed to work but they do not state a size online and I would call to make sure.
There is also this place You can pick the diameter and width but does not state the bolt size. Again it would be best to call. I think this one will give you the best luck.
After tackling a rear differential rebuild, I really didn't expect finding bushings was going to be the hard part. I would think finding bushings by size would be simple or at least everyone would know what size the OEM springs were. I will try to call the companies you mentioned. Thanks.