OK.
Well...
The same thing was also said by the tires dealer in G-6 Markaz, Islamabad. He said that some sizes, brands are rare, and in case of any damage to that rare size / rare brand tire, it becomes extremely difficult to manage it or find in the market. So, its better and preferable to go for the brands and sizes, which are readily and easily available in the markets.
One should never even think of buying them. Such tires can pose very serious risks.
Yes, even 500-510 will be acceptable to me.
Rightly said by him.
May be, there is not much demand of this particular size here.