The American company Humminbird is the leading producer of Fishfinder devices. Other companies such as Eagle and Garmin also make well regarded Fishfinders.
the Garmin Sonar Fishfinders have several features, some I found listed include: automatically adjusting sonar sensitivity, automatic and manual zoom, and a depth reader. I'm sure the features vary by model.
One of the highest rated GPS fishfinders is the Garmin echo 150. You can purchase this item from retailers such as Amazon. Once on the page, type "Garmin echo 150" into the search field at the top of the page to bring up the item.
The cheapest Garmin Fishfinders with color display is The Garmin Fishfinder 160C. Here is a link where you can purchase this product and many more alike. http://www.amazon.com/s?ie=UTF8&index=blended&field-keywords=cheapest%20Garmin%20Fish%20finders%20with%20color%20displays&tag=fstfx2-20
You cant go wrong with a garmin Lawrence or humminbird. I prefer a humminbird just preference, but they all have great features. Your best bet would to be get a fishfinder/gps combo trust me on this one they are great.
A Garmin charger can be bought new from websites such as BestBuy or Amazon. You can also go to eBay where you maybe able to find a Garmin charger second hand and cheaper.
I think that Garmin and Magellan are the best GPS devices. I bought a high-end Garmin device for under $200.
Amazon.com has several Garmin GPS systems for you to buy. I recently bought the Garmin eTrex Waterproof Hiking GPS. I paid around $119 for it. Rice's range from $90 to $350.
There is no immediately available technical comparison of the accuracy of different fishfinders. However, to help ensure that you make a purchase which you'll be pleased with, it's good to check out user reviews on reputable seller websites. For example: http://reviews.walmart.com/1336/4660395/garmin-fishfinder-160c-reviews/reviews.htm Look for sites with an established reputation to avoid spam postings.
One can purchase a Garmin GPSMAP 60CSx Handheld Navigator from a number of retailers. They can be bought from 'Amazon', 'eBay', 'Walmart' and 'Best Buy.
A used garmin, if bought in good condition, should work just as nicely as a new one. You should make sure that it is updated before you use it. I highly reccomend Amazon with this link: http://www.amazon.com/Garmin-Electronics
That should be covered in the user manual that came with it.