I just took a look at the 'Outfitter' for the Ludwigs oaks, and it looks like you can order an 18x12, 18x14, or 18x16 size bass drum.
https://www.ludwig-drums.com/en-us/ludwig/products/drumkits/classic-oak
Too bad there's no dealer around here to talk to. :(
Without that, ideally, you could...