What is the highest nicotine strength available?

This depends on what device you are using and the type of e-liquid as well. 

The highest nic strengths available these days are probably in nic salt pods that can go up to 50mg and sometimes even higher. These work with low wattage devices, like pod mods. In general, if you want to vape at high nicotine strengths, you will need a low wattage device. Vaping 50mg vape juice on a sub-ohm device is simply not a good idea. 

Another factor here is where you live. Some countries put restrictions on nicotine strengths. For instance, in the UK, the max nicotine concentration in e-liquid is restricted to no more than 20mg.

As mentioned above, you can also DIY your own e-juice. But this is recommended for advanced vapers only and, even in this case, it's strongly advised not to go above the  50mg nicotine level.