Mario Kart Wii

There are some that require you to race ghosts before you can get them. If it says there is no other way, sorry, but you have no choice but to suck it up and race them.

You can unlock everything to my knowledge by doing a certain amount of races. As for what those numbers are, I haven't a clue, sorry D;

yes. I got some by winning online races

On a few of the the grand prixs you get one

Bonus bikes
Successfully complete the indicated task to unlock the corresponding bike:

Daytripper: Win 2,700 races.
Dolphin Dasher (medium): Win the Mirror Star Cup. Alternately, win 3,750 races.
Jet Bubble (small): Win the Mirror Leaf Cup. Alternately, win 3,900 races.
Kamek (small): Defeat all the Nintendo ghosts in the Star Cup.
Magikruiser (small): Play on eight different courses in Time Trials mode. Alternately, win 900 races.
Phantom (large): Win the Mirror Special Cup. Alternately, win 4,050 races.
Quacker (small): Win the 150cc Star Cup. Alternately, win 2,400 races.
Shooting Star (Twinkle Star, large): Win the 100cc Star Cup. Alternately, win 1,500 races.
Sneakster (Nitrocycle, medium): Get a "One Star" rank or better in all 100cc Wii Grand Prix Cups. Alternately, win 3,300 races.
Spear (Torpedo, large): Unlock 12 Expert Staff Ghost in Time Trials mode. Alternately, win 3,600 races.
Zip Zip (Rapide, medium): Win the 100cc Lightning Cup. Alternately, win 2,100 races.

Bonus karts
Successfully complete the indicated task to unlock the corresponding kart:

Aero Glider (large): Get a "One Star" rank or better in all 150cc Retro Cups.
Blue Falcon (small): Win the Mirror Lightning Cup. Alternately, win 4,200 races.
Cheep Charger (small): Get a "One Star" rank or better in all 50cc Retro Cups. Alternately, win 1,800 races.
Honeycoupe (Dragonetti, large): Win the 150cc Lightning Cup. Alternately, win 3,000 races.
Jetsetter: Win 4,500 races.
Piranha Prowler (large): Win the 50cc Special Cup. Alternately, win 600 races.
Royal Racer (medium): Win the 150cc Leaf Cup.
Sprinter (B Dasher Mk 2, medium): Unlock 24 Expert Staff Ghosts in Time Trials mode. Alternately, win 4,650 races.
Tiny Titan (small): Unlock one Expert Staff Ghost in Time Trials mode. Alternately, get 50 Wi-Fi wins or win 1,200 races.
Turbo Blooper (medium): Get a "One Star" rank or better in the 50cc Leaf Cup. Alternately, win 300 races

Win a certain number of races,or unlock a certain number of fast staff ghost(expert staff ghost),or win a certain cup,and win a certain cup with at least one star rank or higher