As you have found, there isn't a whole lot out there like OEM. I have been told that you can get replacements from Mazda, but they are a fortune. Your best option is to find an upholstery shop that comes recommended, and have them do a custom re-cover. They will usually use your old covers to make a pattern, and then make new covers in whatever fabric you choose.
I am new to the area, so I can't offer any specific shop recommendations. If you have any friends with old hotrods, start there. They tend to know the shops that can do one offs and pay attention to details.
On a side note, this is something that I have considered doing more then once, and I suspect you and I aren't the only ones. If you decide to go this route, you may want to ask them if the price would be reduced significantly if they were making more than one set. It is very possible that if they sold set 1 for $600, additional sets would only be $300 since the pattern work was done. If this is the case, you could save a lot of money by rounding up a few of us. In my WAG example, it would go from $600 for 1 set, to $450 a set for 2, to $400 for 3, $375 for 4, etc.