Congrats on the new house!
You should be able to contract a carpenter or woodworker to make the letters for you. Just go through the yellow pages and make a few phone calls to get some quotes. Be sure to tell them how large you want the letters, etc.
You might want to contact other DST chapters with houses and ask them how they got their house letters, too.
My fraternity's pretty easy since our 'Greek' letters are also pretty basic 'English' ones, too.
