Where to find List of FPGA OpenCL libraries available/implemented

First and foremost you should know that using OpenCL on FPGAs is a lot less streamlined compared to CPU and GPU right now; much of the burden is still on the users' shoulders. OpenCL only works on a few FPGA boards for which the manufacturer provides an OpenCL BSP. For other boards, unless you want to create the BSP yourself which is next to impossible for somebody who doesn't have years of experience in using FPGAs, OpenCL cannot be used. If you want an ARM + FPGA SoC, your options will be even more limited (but they exist, I prefer not to advertise for anybody by putting board names here). Zync is a Xilinx product and is not going to work with Altera's OpenCL SDK. Snickerdoodle also doesn't support OpenCL. You should look for Altera's board partners who offer OpenCL support.

Regarding OpenCL libraries for FPGAs, there is pretty much nothing official from Altera/Intel. There is probably not much worth mentioning from the community either because OpenCL on FPGAs has still not become that wide-spread (though hopefully it will). Probably the closest thing you can get is a set of code examples provided by Altera. You could probably port existing libraries for CPU/GPU to Altera's SDK, but the performance will be terrible and you will have to optimize them by hand which would need a non-trivial amount of effort.

I strongly recommend taking a look at Altera's (Intel's) OpenCL Getting started Guide, Programming Guide and Best practices Guide; that should give you some idea of how things currently work.