One of the important challenge we faced was to procure or order parts for prototyping some new design. Regularly we need to get new parts (which are not in our inventory :) ) as for product level development we need to iterate for making it robust and cost effective. So we have used following channels:

1. Element 14 (Works very good in India)
2. DigiKey (or Mouser)
3. Aliexpress (the quality is decent, but sometimes the products are faulty)
4. Analog Devices
5. Texas Insturments
6. [India] Lajpat Rai market, Chandani Chowk, Delhi
7. [India] Nehru Nagar Market, Kanpur, India

Most of the ICs can be ordered as samples from the manufacturer website like Analog Devices and Texas Instruments. It was really helpful as they ship using very fast modes.