Some alternatives to going through crosman., but yes, they should have 24" barrels. http://www.mellonair.com/http://www.mountainaircustomairguns.com/New_accessories_page.htmhttp://www.cothranmachine.com/http://www.gmaccustomparts.com/http://www.crookedbarn.com/There are more, but I don't have experience with them. I have bought from all of the above and would do business with them again. Also, do a search on Ebay for "Crosman"... there are usually a bunch of barrels for sale and the occasional steel breech (Mellon's prices are generally better than what I see on ebay).
Flat toppoing your valve is very doable, if you dont have a lathe or drill with a chuck big enough to fit the valve, careful work with a file and some wetordry sandpaer can get the job done. Just be careful and dont damage any of the o-ring groove edge.