There are two, non-surgical treatments that are commonly used for ferrets with Insulinoma.  Neither of these are a cure.  They simply help to control the symptoms.

Prednisone (Prednisolone, PediaPred, Prelone)

This is usually the first drug that is given to a ferret with Insulinoma.   Prednisone acts to increase peripheral blood glucose concentrations.  Dosage ranges from 0.5 - 2.5 mg/kg, twice a day.   Pred, as it is commonly referred to, is relatively inexpensive and initially works quite well and there are few side effects.  Ferrets do tend  to gain weight when on prednisone.   If using the liquid type, make sure you do not use a generic form which can contain alcohol.  Also, there are different concentrations so if you get it from a local pharmacy, make sure they understand it's for a small animal and get the dosage information correct!  As the disease progresses, prednisone may no longer control the symptoms even at the maximum dosage.  Prednisone is converted to Prednisolone in the liver, and some ferrets may do better on Prednisolone than Prednisone.

Diazoxide (Proglycem)

Proglycem is usually prescribed when Pred no long works.  This drug can be added to the Pred treatment, lowering the Prednisone dosage in the process.   Proglycem dosages start at 5 - 10 mg/kg, twice a day and can go up to a total of 60mg/kg per day. Proglycem works by inhibiting insulin release from the pancreatic beta cells, decreasing cellular uptake of glucose and promotes glycogenoloysis and gluconeogenesis by the liver.  Side effects can include anorexia and vomiting.  From personal experience, Diazoxide does not appear to work well for all ferrets. 

The major drawback to the use of Proglycem is the cost.  A month supply for one ferret can cost as much as $130 dollars.