I don't have access to celery root (celeriac). What can I use instead? Are there any suitable alternatives?
From Cook's Thesaurus
turnips OR celery ribs (weaker flavor) OR parsley root OR (in salads of grated vegetables) carrots + dash celery seeds.
For a soup, you could probably use parsnips, rutabega (aka swedes), or even potatoes in place of celery root. Carrots would probably be fine too, but if you're puréeing the soup, they'll probably affect the color.
If you would like to find celery root, you might try an asian market. In the past, I've had success at a chinese market when all of the other local stores were out of celery root (in San Francisco).