The converters I used (I used one for tacho and one for speed), I got from here
The exchange rate's not so good now - mine cost me less than £20 each I think and now they're more like £30. Delivery from Australia (!) was less than a week iirc. Assembly was straightforward (as long as your soldering is OK) and the instructions were good. They've been trouble free. Just got around to tweaking the speedo the other day finally to set it up properly versus GPS (was about 10% low). I bought their recommended boxes but either they sent the wrong one or their recommendation wasn't too good - it didn't fit. A trip to Maplins sorted that.
There are similar things available in the UK but they all seemed more expensive and less versatile - this one lets you go 0% to 100% change in either direction.