What can I use as a placebo tablet for Medrol 16mg? I need a inert
substance. Not something that will interfere with blood glucose
levels(eg sugar). Not
something that will interfere with oxidative stress(eg vitamin). I
don't care about the different appearance of the tablet. How can I
obtain the placebo tablet?

Capsules filled with titanium dioxide?

you can't substitute capsules for tablets if this is a blind study,
that could insert bias.  Do you have access to a tabletting machine? 
perhaps a local pharmacy school could help you?  when i made tablets
in pharmacy school we made blanks, ie placebos (it was cheaper this
way)... perhaps you could see if a local university could donate to
you their blanks.  If you have a tablet punch, you can make your own,
with any sort of pharmaceutical binder and filler using either wet or
dry granulation, ya know whatever suits your needs better.  here is a
list of excipents that cou can use:

http://www.pformulate.com/labclass/excipients.cfm

If all you are worried about is effect on BGL, and need something
right away, some multi vit's would work, like a folic acid supplement
looks like a medrol.

or if appearence means nothing, like rhenium said, empty capsules are
easily available over the internet and you can fill them with TiO2 or
any other inert excipient listed in that link...