Both my A4 and A3 prints easily slot into ready made frames. These examples are poster frames from John Lewis.
This frame is suitable for an A4 print,
this frame is suitable for an A3 print.
If you purchase a frame elsewhere, just remember that A4 and A3 refer to the aperture size. The overall frame size will be larger than this.
Alternatively, take your tube to a profressional framer for a bespoke finish.