keypower will sell you a dc-dc power supply for ~$200... no modifications will be allowed.
building a "sproggy" power supply will take you ~$50-$100 (depending on part availability) and will take probably a couple hours to build and test.
go to the for sale section and look for dc-dc power supplies being sold by MASTERO. he has $160 commercial supplies shipped anywhere in the world...
and PLEASE SEARCH HARDER next time. this is a very common topic.
having a custom power supply built to you specific specs as a "one-off" part will probably cost you no less than $400, especially since they will have to guarantee it since it will be sold by a commercial distributor...