Sniper Central is the website I refer to above, they offer long range rifle packages, one of which is based on the Howa action. Their rifle forum features a good bit of discussion on Howa 1500s. They have a "shop" catagory that list all the options available to one who would like to have them build a rifle for them.
As for me, I picked up a 308 heavy barrel varminter 1500 from Academy and put a Meopta scope on it. If one wants a longer barrel you may be dealing with the classifeds or the site above. Legacy Sports may still offer an action with a 24" barrel, but one costs as much as my rifle, and you're still building a rifle around it by adding a stock, etc.