Like Ian SC, I made my own (using a 24V window winder in my case). If you go the DIY route, there are pics of my feed in my album here **LINK** . A variable voltage power supply (0-15V for a 12V motor) is helpful, as it allows you to change the feed rate to suit the job.
I now have a Bridgeport clone so haven't touched this machine for ages. I do remember though that there are power feeds available for the X direction but they cost almost as much as the machine was worth, hence the DIY solution. Depends how rich you are feeling!
If you do a Google search on "power feed mill drill" you will see that generally, feeds for this kind of machine are based on the power feed units that were developed for Bridgeport type machines, modified (fitted on their side) to fit onto the ubiquitous mill drill. So if anything they are more expensive than the original Bridgeport part. they are derived from.
I see that Chronos sell Vertex power feeds (decent Taiwanese brand). For the std Bridgeport application they seem to be about 240 pounds. For the mill drill, they are 270 pounds which sounds about right. **LINK**
Merry