You get Banners from buying "Bundles" in the store, from reaching certain levels, and obtaining high amounts of achievement points. Banners can be worn as hats while in a lobby and used as items in housing.

There are 16, only 5 of which are currently available, banners:

Name How to get Cost Availability
Hypixel "All in One" bundle 309.98$ Not Available
Snow Bunny N/A N/A
Holiday Tree
Holiday Wreath
Heart Bundle 1 85.68$
Guitar Bundle 2 93.98$
Dino Bundle 3 133.98$
Redpool Bundle 4 148.31$
Pengu Bundle 5 156.65$
Crown Obtain 9000 Achievement Points N/A Available
Skull King Reach Hypixel Level 100
Devourer Obtain 10000 Achievement Points
Wall Crusher Obtain 13000 Achievement Points
Deal With It Bunny Obtain 15000 Achievement Points