You can get replacement 20 round mag springs from aftermarket suppliers such as Wolff.
You can use black plastic followers, or adapt the green new style plastic followers for use in a 20, but there's no source I'm aware of for the original alloy followers used in the USGI 20's.
There's no direct source for replacement floorplates for a 20, but you can use a 30 floorplate on a 20 if you cut a notch for the spring catch, or remove the spring catch. 30 round floorplates lock at the back instead of being cut for and using the spring catch like the 20's.
In addition, bushmaster lists 20-round magazine rebuild kits on their website (#8448670-20K) for $5.95.