I suggest referring the customer to TGS' paste tomato pages, starting at:
http://www.tomatogrowers.com/Paste/products/7/
If the customer insists on something close to exactly matching Roma in shape and size, he may be stuck with determinates, which have the disadvantage for a market grower of producing their entire crop within a few weeks. TGS carries a very few indeterminates that are close matches, but not many, and I don't know enough to recommend for or against them.
If matching size and shape with Roma is less important than just having a really great tomato, well, my only experiences with paste tomatos have been with San Marzano and Sausage, neither of which I think would be ideal for your customer. But I've read so many glowing reviews of Opalka, I'd think that might be one of the best suggestions you could make. Let's see what other Tomatovillians have to say now...