There are UK-based sellers offering them for around $10 per plug, and postage to the US would not be that expensive either. Have a look at www.wemoto.com, I've bought mine from there and they do ship internationally. The website will calculate postage for you.
There is no alternative to the specified plug -- only NGK makes them, specifically for Suzuki. Here's a pic of an MR8E-9 (left) and a CR8ES (the standard plug from my old 2006 SV650). As you can see, the MR8 has a projected nose to put the electrodes further into the combustion chamber:
What I can tell you based on my own 2017 Gen 3 SV650: changing the plugs at 7,500 miles is bullshit. The owner's manual that came with my bike has two service schedules printed in it: one for 'Europe & Oceania', one for 'rest of the World' (i.e. the USA etc.
In the Europe schedule, the plug change interval is 15,000 miles, with a gap check at 7,500 miles. The rest of World service schedule says plug change every 7,500 miles. It's exactly the same engine, exactly the same spark plugs. You do the math.
I changed the factory original plugs on my bike at 15,000 miles. The plug gaps were all still within the 0.8 - 0.9mm spec. There was a little wear on the electrodes (mostly the centre electrode which had lost its sharp edge) but nothing serious, the plugs were still perfectly usable with no carbon build-up at all.
You're making me think I should change my plugs... I'm around 30k miles that I know of. I doubt the PO changed them in the 12k miles he rode it before I got it so I'm probably over 40k on the originals. It sure runs good though!