OK, I don't think that you can buy a bottom support that size, but you could make one. Just use a full length piece of hardwood under the drawer that is shaped with an 'outside' dovetail and then put the 'inside dovetail on the very back, bottom of the drawer. You should be able to get almost full extension then.
Your could also, make the sides out of hardwood and then put a tab that hangs down on the very back, bottom of the drawer. This would allow your to pull the drawer all the way out and even tip it down if you wanted when it was all the way out.
If you need pictures, I will have to see what I can do.