Most, if not all depth finder transducers used in the US are made by Airmar:
http://www.airmar.com/
You have to remember a number of things about depth finder transducers:
They are frequency specific--some at 200 khz, some at 205, some a 183, some at 50 etc. So the frequency must match.
The cable is part of the circuit and shielding is important--they can be spliced, but my recommendation is not to.
The end fittings are often unique to the brand of depth finder.
The NMEA 2000 Buss transducers may be using a common fitting (since this is part of the standard). However even though my Lowrance LCX 26 C HD is NMEA 2000, the depth finder circuit is NMEA 0183. We may find that in the future that sounder transducers may be more compatable between brands. For example, the Lowarance fuel flow meters apparently work on the Ray Marine "E" series--both being NMEA 2000.
Same for instrument guages.
At times it is cheaper to buy the transducer from Airmar--or you may get a transducer more suitable for your application than the standard factory unit.
Another hint--is to screw a 3/4" piece of Starboard on the lower transom, and screw any transducers into that, rather than into the hull--it is easier to change or move--and you don't end up with a hull full of holes and water intrusion into the lower transom.
I purchase almost all of my electroncis/chart chips etc on line. Most are drop shipped from the manufacture, so you get the latest updates. Some are downloaded at the local dealers--but if it is an instock or warehouse item, I would be suspicious about not being the most recent update. All of the chart manufactures have the most recent version on the web sites so you can double check your copy. I would be interested to know if Jim thinks that the Platinum chips are that much better than the Gold or silver.
Unfortunately my plotter does not use the platinum series--but I have the C map MAX on another plotter--which I think is comperable...But may not have quite as good bathyspheric data--especially 3 D modeling (or if it does I haven't figured out how to use it). If you are using the 3 D bathyspheric mode--how useful is this for you?