Hardwoods are not sold as dimension lumber but as random length and width of a given thickness. If you really want a 3.5 inch net, you will need to buy 16 quarter wood (16/4 = 4 inch rough) and get it surfaced.
Look under hardwoods in the yellow pages or in the back of Fine Woodworking for other sources. if you order you will probably need to use motor freight since UPS has a 160 pound limit and the length is probably to long also.

Try a full service lumberyard. They often stock oak in sizes like that---I don't know about the length, however.

I can't imagine the cost for these if you can find them. One other question. How the heck are you going to anchor these mammoth things down. Why not make the posts out of 4 seperate pieces of oak so that the center is hallow. That way you can cut a piece of wood the same size as the center but only a couple of inches high, anchor this piece down and then add some glue to this piece and then fit the post over it and nail the post to it, countersink the nails, putty over the nail heads and you're done.

Local wood shops and cabinetmakers night be able to help you with this.

Oak that thick would be very difficult to dry. Best option might be to have one of the shops glue something up for you. Glued up stock would be more stable.

Where in Illinois are you?

What is it you are doing with this kind of oak? posts?

Suburbs of the Chicago area.................but this is for a home in Arkansas. Finaly found a mill company 115 miles away that can do the job for me.
Thanks for all your help and suggestions.

I have three 9 ft places for

My e three3333@netzero.com

Kenneth, did you happen to notice how old this thread was?

I'm frankly surprised this person didn't find sawmills all over Arkansas that would have (or make) oak timbers in any size she needed.