According to Clotilde, all Riccar machines are low shank. That means you can buy a generic low shank walking foot and it should fit. If you are still not sure it is a low shank machine, here are instructions for how to figure it out:
http://www.clotilde.com/pages/which_foot.html
Not all generic walking feet are well-made. If possible, you should buy locally so you can take it back if it doesn't work well. Check your yellow pages for sewing machine dealers in your area and call to see if they have what you need in stock.
If you can't find a walking foot locally, there are many online stores you can order from. Not everyone likes Clotilde, but that is one source. If you Google low shank walking foot, you will find many sources. Allbrands has one that costs twice as much as Clotilde's; could be twice as good, but I just don't know.