Any suggestions on where may I buy? Is online preferred for Laptop HDDs?
You can buy online but buy only from a reputable seller. HDDs are fragile item and packing is important. Once you receive your new HDD, it's usually a good practice to inspect it's SMART parameters. For that purpose, you can download and use "Crystal Disk Info" in Windows and pre-installed utility in Linux.
My personal recommendation are Western Digital Drives. They tend to be on slightly more expensive side but are more durable.