所以用3rd party arm and foot.
or
3d Print it yourself.
The only problem are:
1: Price: Most add on parts are X>$20 USD.
2: 3D printed parts needed to be sanded (not everything is perfect cut) and painted. QC is an issue.
3: Some parts are just addon for Robot mode (that's mean no storing in vehicle mode).
4: Printing material needed to be durable.