I just recently bought cruise insurance. I had a brochure from American Express but went with a different company because the coverage seemed to be a little better (for example, trip interruption coverage kicked in after 6 hours, but with Amex's it didn't kick in until you have been delayed at least 12 hours) and it was less expensive, though not by much. You can e-mail me if you'd like and I'll tell you what company I bought my cruise insurance from. Also, remember that if you buy the insurance within 7 days of making your first payment to the cruise line (i.e; your deposit), they will waive the pre-existing conditions exclusion on the policy.
Our local AMX agency uses Access as does AAA. This company is very reputable but may not be the cheapest but certainly not the most expensive either. Like most major purchases I usually compare 3 products. Try Travel Guard and CSA.