Muscle Boy wheels were in Rocket League's sixth pass. Rocket Pass 6 was active back on March 25. The limited time wheels are stylish, but now are hard to come by. If you didn't get the Muscle Boy wheels from the rocket pass, there's only one way to get them now. If you want a Muscle Boy wheel, you'll have to trade for it.
Muscle Boy Wheels Rocket League: How to Get
Trading is the easiest ways to get any specific item a player wants. There are two difficulties when trading. The first is finding a player with the item you want. The second is making the trade worthwhile for the other player. Websites such as rocketprices.com help players track how much an item is worth in terms of in game credits.
According to rocketprices.com, the Muscle Boy costs roughly between 350 and 550 credits. Players can acquire credits from the in game shop.