Red Oak $2K (Berkshire)
YouTube has split top slab bench is artificial split top. (see link)
Also look to "river table". "Split top table". At least one is a artificial split that looks decent.
Local Woodcraft store features some slab blanks and some have holes and/or splits. Priced in the $600 range in various sizes, but not near dining table sizes.
Trade publication Woodshop News (www.woodshopnews.com) has ads for large slabs. You might contact them. Also Berkshire Products (BerkshireProducts.com)413-229-7919, large selection of big stuff