RFXcom used to be a popular alternative, and was still sold until recently (but very expensive). If you're using it for just X10-RF, and not DS10A type security devices, MR26A might still do tn.com/NooElec-NESDR-Mini-Receiver-RThe job.
At some point, I was going to build an OpenMQTTGateway instance, and build my own custom receiver (can be done cheaply). Depending on the software you use (or deploy previously-mentioned OMG), you could probably just get a
NooElec SDR (~$30), but you would have to dedicate it to this frequency. I believe RTL433 supports it (protocol 22), which is supported via Home Assistant.
In the end, I decided against going this route, and switched to Z-Wave for my door sensors. You just need to look out for good deals if cost is the reason you're sticking with X10.
I may have a W800RF32 somewhere, but can't remember if it still works (most likely does). If you're really want to go that route, I can try to find it, but may not be able to get to it for another week or so.