You can go (within limits) just about anywhere for the IR, but that won't necessarily get you quality. The good schools generally have queues because people want to get quality training with experienced instructors. They also might want a certain school on their CV to assist with job prospects. Some instructors might also be airline pilots who can facilitate in the job hunt if you meet the standard. Personally I would pick a good one and wait. Use the waiting time to get your skills up to speed. Talk to the school you have in mind and find out what bits of the IR syllabus most students struggle with and go bash some hours in your trusty C152 honing those skills so you hit the ground running. Get their SOP's and know them backwards, get their a/c checklists and know them backwards to along with the relevant POH etc. Find out if they use Jepps or Aerads and know them inside out. Read the AIP and find out all about DOC's and approach bans and RVR's and all that sexy Air Law stuff that you thought you would never need after you had done your ATPL theory exams. Get a copy of RANT or MS Flight Sim and practice NDB and VOR tracking until you can do it with your eyes closed.
Not only will you save some money (hopefully) it will all set you in good stead for later when Mr Airline asks you a whole bunch of technical questions about the mighty Duchess/Seneca/Diamond.