In both SP & MP You can buy them for like $1,000 online and they don't take up space as a car in your garage so you can have 6 cars and one BMX Bike if you want but if you are talking about SP
There is a BMX cheat you can use.
Just put the cheat in when you're not doing anything that you'd get an achievement for unless you don't care.
Left, Left, Right, Right, Left, Right, Square, Circle, Triangle, R1, R2
Left, Left, Right, Right, Left, Right, X, Y, B RB, RT
I like using the BMX Bike because it gives you strength and stamina but you can't use it online because noobs take advantage and kill you even in passive mode you are able to be shot and killed or ran over.
If you manage to find a BMX Bike Online there is a couple half pipes near a garge in the Figure 8 on the south east side of the map that's near an LSC Shop & Garage