There are definitely companies that would be willing to cut you a pair of matching mattresses in this size. There are even companies that specialize in foam, and can make and cut foam mattresses specially for the consumer. One example would be Foam Order, which is based here in San Francisco but does business nationally online.

In terms of the cost, you are probably looking at something between $400 and $700 (each) for the foam. On top of that, you'll need to add the cost of covering / quilting (+ taxes and shipping, if applicable). The cost of the foam will depend on:

1. The quality / density of the foam you use. Higher density foam costs more, but holds up much better over time.
2. What portion of the mattress will be memory foam. No "memory foam mattress" is 100% memory foam (there would be no support), so with an 8" construction you'll generally want only 2-4" of memory foam on the top, with the rest being a firmer layer of polyurethane foam for support underneath. Memory foam is 2-3x as expensive as regular polyurethane foam, so having a 6" base with 2" of memory foam would be cheaper than a 4" base with 4" of memory foam.

That said, would you be able to accept a length of 80" rather than 78"? If so, your specifications would align perfectly with an existing size called "queen split". Queen Split is not one of the most commonly available sizes, but it's used in order to make queen-size mattresses work with a dual-adjustable base, so is becoming more common. If this more standard size would work for you, it would open up more options for you and would probably save you some money.

Another thing to consider about building your own mattress is that you are taking some amount of risk in terms of fitting it to your personal support and comfort requirements. If you are able to do this locally where they can show you what it will feel like, that would remove a lot of the risk. Otherwise, you'll want to make sure you are buying someone that really knows what they're doing and that you can really clearly communicate your preferences to them.