The 285/40 tires are not possible to obtain in North America in a VR rating. The only source is a very occasional production run by Yokohoma in Europe. The Svenska De Tomaso Klubben buys up these production runs, stockpiles the tires and then offers them for sale.http://www.detomaso.nu/
Roland Jaeckle in Germany also usually has some for sale:http://www.jaeckel-partner.de/en/detomaso.html
Be careful if you happen to find some advertised in North America, because they will usually be very old and, therefore, unsafe. There are threads here on this forum that explain how to read the date codes.
Be ready for a shock, though, because the price, after you have paid all the shipping, import duties, taxes, brokerage fees, etc is stratospheric. But don't worry; your kids probably didn't want to go to college anyway...