Brother, as I mentioned earlier, each car has different specification mentioned on the Driver Side B-Pillar. You have to refer to that to find a sticker with the suggested tyre size and front-rear inflation pressure recommended.
The B-Pillar
The Tyre Specification Placard (Aka Sticker)
The Information
For finding out rim offset, I am afraid you have to follow the already mentioned procedure in post 1:
So the question comes, "I as a buyer don't have enough time to go through all this on a retail shop. Its time consuming and seems insane"
Answer: No. It isn't. Just take along a measuring tape and it takes only 30 seconds to measure and calculate rim width and offset.
My appologies for not being able to help you further. I, being a user of a handful of vehicles, cannot keep track of every car on road. As the owner, you have to do some homework yourself.
Cheers!!
^Brother, as I remember correctly, alto never came with OEM alloys. So your best bet is steel wheels. Or else a chinese can work, but i highly doubt you can get your required size maching your steel wheels.
However, if you try to search for some rims taken of JDM KEI cars, that might help you. They come in some odd sizes, not generally available in market