You might want to try to find a Chinese chip company that still sells the 68040's, yes, there are still come companies that make new chips of this old design.

If you want to stick to strictly vintage chips, I would imagine you'll have a hard time finding a 40 Mhz at a reasonable price, from what I know they didn't make as many of them and they are significantly harder to find than their 25 Mhz & 33 Mhz siblings.