Hi Jasmeet
RFID tags come in various frequency range; for building projects we generally use 125 KHz modules but there are no limitations of using other frequencies, as such. You can easily get them online on Amazon, ebay etc. or if you live in delhi you can visit Bhagirath Place market in Chandini Chowk and purchase it from there.
Yes you can definitely design one, but it would require a detailed study and knowledge to do so. Check this link
hackaday.com/2010/04/19/build-your-own-rfid-reader/