The carriers handle device warranty (not RIM, manufacturer of BlackBerrys). That is the arrangement. So, you need to talk to your carrier, but I'm almost certain a broken screen is not covered.

Insurance is different from a warranty. If you have insurance, you need to talk to the insurance provider, unless you've been instructed to work with the carrier.

As far as Do It Yourself, I can't advise, except to say there are threads on this board and others in which suppliers are discussed. Until someone comes along with suggestions, you probably know the drill: search for sources, compare prices, read reviews, search the internet for independent information on the sources.

One other bit of advice... There can be several different screens used on a BlackBerry over time. Make sure you use the same one.