We have found some good deals recently on 450 v. electrolytics at Mouser (individual value caps, you can use a terminal strip to mount them on and replace the cans)
Just search at
http://www.mouser.com rather than using the paper catalog.
Am not sure if they have 600v mylar caps as we have not had to re-stock those yet but we have found Mouser gives the quickest service and a true wholesale price on what we have ordered from them.