RCP57 has your answers if you want the Firgelli ones
Where to get them:
http://www.rctruckandconstruction.co...&postcount=105
What size:
http://www.rctruckandconstruction.co...2&postcount=99
If you have money to spare, you can go for the CTI ones from Germany which a lot of guys on the German forums use. (prices include the 19% sales tax. So you can divide price by 1.19 if you are not in EU)
For the bucket movement:
http://www.cti-modellbau.de/CTI-Hubz...0-185-328.html
The arm movement:
http://www.cti-modellbau.de/CTI-Hubz...5-238-239.html
CTI also has a complete kit for the conversion with complete aluminium frame, engines, wheels etc.
http://www.cti-modellbau.de/Baumasch...4-277-287.html