Ebay is full of parts for the RS4, Tower Hobbies is also a great place to shop. The prices are decent and the shipping is really quick. Overall I have done really well on on Ebay finding what I need.
You can put some rally tires on your RS4 and get a little more ride height, try changing suspension setting to gain some ride height. You will not have a full off road RS4 but you will have a little more clearance.
Look on Ebay for a rally car, you will be suprised how cheap you can buy an RTR in good shape from someone looking to get out of the hobby.
Good luck!