I am looking for a good, trustworthy mattress company that makes custom size mattresses, specifically for an antique bed frame. I believe it takes a 3/4 size mattress. I need a firm regular (innerspring) mattress, not foam/memory foam. I am in Seattle, but presume we may have to order one from out-of-state. Don’t want to break the bank cost-wise, but need to find a reputable store. Any advice? The bed will be used daily, it’s not a guest bed. Thank you for any replies!
HI Mslivluvlaf,
Welcome to the Mattress Forum! 
Subject to first confirming that any retailer or manufacturer on the list that you wish to visit is completely transparent (see this article) and to making sure that any mattress you are considering meets your specific criteria and the quality/value guidelines I linked earlier in this reply, the better options or possibilities I’m aware of in and around the Seattle/Tacoma area are listed in post #2 here.
While this list is not complete, it does have some local manufacturers who produce custom-sized innerspring mattresses, and some are in the more affordable price ranges. You might also be surprised at how many of the licensees for some of the “smaller” top-15 name brands will produce custom sizes. It would come down to phoning either the retailers on this list or other local retailers and simply as if they can get you 3/4 sized mattresses.
There were two different sizes of 3/4 mattresses made. The most common was 48" x 74". Then there was a less common 48" x 72" version. Be sure to take careful measurement with your antique bed set, and also be sure to make sure you are using proper slat reinforcement for your new mattress and foundation. Old bed sets usually used only three cross slats, and you’d want at least five with some sort of center reinforcement for any newer style mattress. You may also end up getting a lower profile foundation for your new mattress set, as the antique beds were designed for use with thinner mattresses and box springs.
I think you’ll be able to find something closer to home for your specific request. 3/4 is a “common” odd-size, if that makes sense.
Phoenix