Generally tubes are not specific to one size tire, our rear tire is a 180/17 so anything from a 160 to 200 will work. Same for the front, it's a 120/18 so any tube from 100 to 140 will work.
I have ordered these tubes for our classic to have as spares.
If you're in the USA, here's where I got them, if not you have part numbers now.
http://www.bikebandit.com/tires-tubes/motorcycle-tubes/metzeler-motorcycle-tube
Part Number: 174669
Metzeler Motorcycle Tube
17, 190/55-17, 200/50-17, 210/50-17, 170/55-17, 180/55-17, 160/70-17, 160/70-190/50-17, TR-4 / TR-6, TR-4, Center, Standard, 8mm, 10mm
$30.74
Part Number: 174662
Metzeler Motorcycle Tube
18, 4.00-18, 4.25-18, 4.60-18, 120/80-18, 130/80-18, 140/80-18, 110/90-18, 120/90-18, 130/90-18, 140/90-, 110-140/90-18, TR-4 / TR-6, TR-4, Center, Standard, 8mm, 10mm
$37.95
And while we're talking tube repairs, this is a very compact stand I've used on various bikes from lightweight dirt bikes to R1200 BMW's for roadside repairs without a center stand.
Enduro Star / Trail Stand
Safe Travels!