I would consider most of them provide XML. They probably do not mention it on their website but I am sure that it is provided. I good starting point is to take a look at http://www.sbrlines.com as all the books listed here have to provide XML for SBR to be able to show the lines.
Using XML is quite hard and not for the novice webmaster and this is probably one reason why very few sportsbooks mention they offer XML, yet I am sure most of them do.
Just my two cents,