There is not supplement exactly like metformin.

Metformin does several things in the body to control blood sugar.

One is that it stops the liver from making too much glucose, during fasting periods. I don't know a supplement that does this.

At times, some nutrient will be highlighted as equal to metformin, but that usually does not pan out in the long run.
For example for years d-chiro-inositol was suggested as doing this. Well it does work, in women with PCOS who are infertile, and when used "like" metformin it allows for ovulation to restart.
But it does not have the same "power" on blood sugar.

Lipoic acid will help alot. I'd suggest you use the new form,
Stabilized R-lipoic. It is more soluble and can work at much lower doses. Benfotiamine is good too.

here is the supplement thread:
http://neurotalk.psychcentral.com/thread121683.html