How I can purchase app in different region

You cannot.

To use a country's AppStore, you must

1] be in that country;

2] have a credit/debit card issued by a financial institution in that country; and

3] have a verified billing address in that country.

Suggest you contact the developer of whatsapp and ask him to allow its

sale in your country's AppStore. There are countries where the government

blocks the sale and use of certain apps; is that the case where your are?

Whatsapp doesn't work on an iPad in any case. Whatsapp is only compatible with the iPhone. So even if it was available in your country, you couldn't use it.

Regards.