From Sonic Retro
In Sonic Adventure 2, the Booster is an upgrade to Miles "Tails" Prower's walking mech. It enables the mech to hover short distances after jumping. It is necessary to complete the level it is located in: Mission Street.
To get the upgrade, proceed through Mission Street until encountering a large gap in the road that can't be jumped across with a line of rings. Turn around from the gap to find the Booster on a platform behind a small road. Be careful when getting it as the road will collapse, requiring you to fly after you pick it up. Fly over the large gap to continue on with the level.