Where are you planning to travel? I don't know about every continent but I do know that in Latin America the 17's are easier to find than 18" sizes. Stick with what you've got. Your bike uses a 130/17 ... but you could probably use a 120/17 or 140/17 in a pinch. A 130 is a very common size ... used on KLR's, DR650, some XR's, XT's, BMW F650 and many others. These days it's readily available in most Latin American capitals at the good shops. Not a problem. The 18" in your size ... rare, hard to find.
There are a few Transalp experts here on HUBB who've done RTW travel on that bike, know it inside out. I recommend contacting them, learn what you can about set up and prepping the bike.
If you are going places where no rear tires are available, then carry a spare on the back. Many do. The 21" front is very common, available just about anyplace tires are sold. PLAN your tire changes well ahead. KNOW how long your rear tire will last. (Usually two rears for one front) KNOW where you can find your next tire! (basic, no?  )
Pack light, have fun!
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