You can buy NFT's from Marketplaces. There are several marketplaces that you can browse and make purchases using your crypto wallet. Some like Superrare focus on Art and feature rare NFT's by established artists others sell a vast range including art, music and domains like Open Sea. You can sign up to different ones and purchase from as many as you like and can afford. All your NFT's will show up in your wallet so you'll be able to view your whole collection of various purchases from different marketplaces there.
Other Popular Marketplaces:
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17 Marketplaces: Where to buy NFT's - Builtin.com