I don't think you are going to find much evidence for IVIG and idiopathic SFN in the scientific literature. You might get lucky and find a case study, but understand that it is considered a low level of evidence. Your neuro is going to want to see several journal articles with a higher level evidence, especially if he is a stickler.

http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0022268/
This systematic review pretty much says that the evidence that IVIG works for neuropathy was insufficient other than for conditions like MS or GBS (and a few other conditions as well). This is likely what your neuro was referring to.

You might try to sift through Cochine Review; doctors take opinions from this review very seriously.
http://www.cochrane.org/

As previously mentioned, Google Scholar is a great way to find full text journal articles.
https://scholar.google.com/scholar?h...%2C50&as_sdtp=

Not only should you search pubmed, but you should search all NCBI databases.
http://www.ncbi.nlm.nih.gov/gquery/?...europathy+ivig

Some of the two most useful NCBI databases will likely be Pubmed Central and Pubmed Health. Learn to use the Advanced Search/Search Builder to optimize what you are looking for in any given database.