Fill for large raised vegetable bed?

No point - grass clippings and hay will rot down and the soil level will drop as and when they do.

Also you will have lots of seeds from the hay. I have just made raised veg plot (from long garden to veg plot my blog).

We used top quality top soil and well rotted manure from a local source. This was dug together and will be mostly left until next year.

You will find that the level drops as the soil settles to it's own level. Ours looked really high to start with, but now its had lots of rain it is much lower then when first put into the bed.

for top soil you will find some free, if you are prepared to bag and carry yourself or you can have delivered. Some companies deliver in bags, or like ours it came in skip. If you go with the skip, remember to get it as close to your bed as possible, it's backbreaking moving it to the bed.

Also, if your part od the States has hot summers, all of that raw organic matter underground will go aerobic and release gasses that are toxic to the roots.