It's about $1,200 for a board with that GPU - and though laws vary from location to location, no - there is at least in the US, no regulation regarding the availability of replacement parts for any set period of time.  And with notebook computers, after three years (when the largest number of warranties have expired), generally manufacturers in fact do not stock replacement parts.  

On some models, Dell ceases stocking parts even sooner than that - figuring that in case a warranty repair is needed, they simply swap a refurbished system rather than replace parts.

Unfortunately you can't just change the GPU on your machine as it's soldered to the motherboard along with the CPU. You can do a motherboard upgrade to a 1080 but it won't be cheap. Check The Parts People to see what they have.

By law, Dell have to supply replacement parts for their machines for 7 years even if a machine is out of warranty so you can try them but it will probably be extremely expensive, if they are even happy to help at all. 

Alienware has a Graphics amplifier you can get think you can go all the way up to 2080.