What would you do with it.
If you don't find one already made, you would have to:
First you'll need a very large enclosure.
Second you would have to know how to calculate its dimensions an the port.
Third it may not have an efficiency compatible with the rest of your speakers.
Fourth you would have to buy or build a cross-over network, for what you would have to know what cut-off frequency to use.
Fifth you would need an additional amplifier for it.
There is more, but not as important.
There are two main ways to obtain very low frequencies: (1) Using a large cone with short travel, or (2) using a small cone with long travel. If properly designed, the second may behave much better mainly with tight bass signals, because of its lower mass.
Only fool and half mind people will sell an 18 inch home theater powered subwoofer for 250 dollars. You have to spend at least $400 for a used one also.Hope this helps....Byeee !