First, let me clear something up. FixYa, does not stock nor, sell any parts for anything. However, in most cases, we can tell you where to find them.
Unfortunately, DESA International went bankrupt several years ago and is no longer in business. However, some parts are still available thru Mater Parts Distributors. You will need to have the correct Model Number of your heater to find the blower you're looking for. It isn't a case of one size fits all.http://www.masterparts.net/
Having been a DESA/Vanguard dealer for many years, I am concerned about the "continued heat build up on one spot on the roof
." By "roof", I'm assuming that you are referring to the top of the heater cabinet. The heater cabinets do get hot, that's normal. But, if it's getting extremely hot, then that's a issue, that a blower is not going to solve.
The question then arises, what is causing this extreme heat build up in just one spot of the heater cabinet? Has it done it from the start? Has the unit been serviced and checked annually? Was it professionally installed? Is it a Natural Gas or Propane unit? What color are the flames, when it's burning? All these things play a role in what's causing the problem and should be addressed, before a blower is installed. With Vent-free/Unvented Gas heating appliances, you should always err on the side of caution and safety.
I sincerely hope that I have helped you. Please let me know. Thanks.