March Hare, all of these filters are considered (for lack of a better word) "questionable" products, so you aren't going to find many head-to-head comparison web pages or multiple shopping-vendor recommendations. These things are basically flying under the radar and don't want to draw unwelcome attention to themselves.
There are perhaps a dozen such devices remaining on the market, discussed heavily in related threads here on AVS. Google the site for "video filter" and look for the most recently-active threads. The Grex has been the most heavily-promoted (by its mfr) filter, so it scores the most mentions on websites. It does seem to work, although depending on the video material and your hardware it sometimes needs its tiny switches to be adjusted several times until you get an image you like out of it. It can be a bit fussy for some people, but its now the default choice for those who don't have the patience to sift thru endless posts to learn about other options.
The Sima CT-2 is an earlier unti that was mass-marketed thru Best Buy, etc, until Hollywood had it pulled. Before disappearing it was updated to the CT-200 model, which had marginally better quality control. If you find one on eBay for a good price, they're great little boxes that hold onto their resale value. Like the Grex, depending on your sources and hardware the Simas either work perfectly out of the box or need to have their mode switch played with. Here and there you discover a setting that looks awful on some material but great on others. There were followup Simas marketed without the Sima name, I forget now but they may have been the DiMax 5000 and 7000 mentioned by others here.
There's a custom-built filter made by an AVS member, its expensive but reputed to be quite good especially if you need to preserve 16:9 formatting flags. A variety of others are sold by assorted overseas suppiiers and can be found via sites like World Import or 22electronics. Other options would be an old Polaroid recorder used as a passthrough filter, or a full-fledged miniature TBC such as the AVT-8710. A daisy-chained recorder or TBC is less likely to require adjustments or cause image artifacts than a filter, and can be a better value depending on the price and your needs. Depends on what you can afford and whats available at the moment. None of the possible solutions are what you'd call "cheap", they start at $80 and shoot up to over $250 in some cases. Used Simas run about $100, a used Polaroid $80, etc.