You may be able to find vegetarian Grana Padano style cheese, but I don't think you will find the real thing in vegetarian form.
This is because the making of Grana Padano is still a traditional process and true GP cheese is only produced in certain regions of Italy. It in fact has P.D.O. (Protected Designation of Origin) status, which means that only those cheeses produced in certain regions and using certain methods can legally be called Grana Padano. I imagine one of those methods is using animal rennet.
I would be surprised that there isn't some form of substitute available in Italy, as they have the highest proportion of vegetarians in Europe at 10%. I get the feeling, however, that they are perhaps less zealous about it than others if it means giving up good cheese.
The only suggestion I have found as an alternative to GP as something to sprinkle on pasta etc is nutritional yeast powder, which apparently has a similar nutty, creamy taste. Not ideal, but if you make the choice to be veggie you have to take the rough with the smooth!
I see from your profile that you live in the UK; we are lucky in that we have a lot of fantastic small local producers, so it may be worth looking up some local dairies or cheesemakers and seeing if they can offer an alternative.