The kind of buyer you are looking for will vary depending on the level of work that needs to be done. Is it a cosmetic fixer or does the home need be be over hauled?
1) My agent. Try to find realtors who have investors/flippers as clients and let them know of your properties especially if available before listing.
2) Driving around my focus neighborhoods / MLS. Be sure to put fixer keywords / checkboxes.
If you're looking to build up such a clientele, the best thing you can do is show you know how to work the numbers and only bring really promising projects. That means you need to know the likely sale price, the cost to remodel or expand in rough numbers, and the likely value afterwards. The other thing you can do is show how hard working you are at farming a neighborhood. One friend of mine has an agent who contacted every run down property owner and found one resulting in a transaction. And in another case, an agent contacted every property owner in a certain filter whose listing had expired within the last year.
My other recommendation if you're looking for clients is to call every general contractor you know. The last couple we worked with said they are doing more spec homes than ever.