Calcium citrate is easiest on the body, and easiest to absorb. Should be in a vitamin carrying vitamin D3, which helps calcium absorption -- most Crohnnies need vit D, anyway. Dr. Weil most recently recommended at least 1,000IU daily of vit D. Make sure the vitamin doesn't have magnesium, which can cause diarhea and also irritate the GI tract. Vitamin D is a fat-soluble vitamin, so you could try eating a rice cake with butter, peanut butter, or cheese. Don't need a whole meal...just a snack.
Your doc is probably worried iron might upset your stomach...however, s/he probably doesn't realize that food and calcium interfere with the absorption of iron. (Most docs are clueless.) You could try Solgar's "Gentle Iron."
The best way to take iron, for maxiumum absorption, is only with orange juice, no sooner than two hours after eating anything (or taking calcium, a med, or another vitamin), and also no earlier than one hour before a meal or snack (or calcium, med, other vit, etc.)
Oh yeah, and don't take more than 500 mg calcium at a time, because your body can't absorb more than that all at once.