Namaskaram,

It depends on the goal(s) of your worship and planned level of service for this murthy.

If you seek granite/stone deity, then getting a deity from Mayapur will be easiest. Usually one of the devotees at the local ISKCON temple can help you out.

It is easier to acquire a deity of Ugra Narasimha in the North then South.
In South India, worship of Ugra Narasimha is in the household is scarce due to fear of repecussions if you make mistakes in pooja, etc.
Hence Lakshmi-Narasimha Murthy is usually more commonly worshipped form. Brass Deities of this form of Lord Narasimha are easy to acquire in South India.

If possible, you can also keep Yantra with deity to aid in energizing your pooja. Usually lakshmi-narasimha or (maha)sudarshana yantrams.

Finally, as a general rule, the height of the deity should not be greater than the distance from the base of your palm to the top of your middle finger.
It is regarded that if the deity is bigger then this, there will be some difficulties in the household after some time [e.g financial or relationship problems].

Hope this is of some aid.