Hi Owen
Tricky one there. If you hang around the standard places such as internet forums, conferences, training seminars, etc. you'll find that there are an awful lot of outfits claiming to be able to source properties.
Of course some are excellent - they have first rate knowledge of their areas and are competitively priced. However, some will add far less value, and are to be avoided. I personally got in to all this through a recommendation of some sourcing companies in the North East from a colleague. Word of mouth, from people I trust (plus my own research) is pretty much how I find and follow up leads.
In terms of the agents I use, I look for successful lettings agents who can source (just my preference of course). For me the risk of voids/ having my property unlet for long periods is paramount in my decision. If I can get the agent on my side early on and give them my sourcing fee then I anticipate they try harder for me when my property is in tenant difficulties!
Where abouts in the country are you looking to buy Owen?
Best regards
John