If booking Kidani, OKW, or SSR, you can pretty much book any time if you're going off season (these are the three biggest DVC resorts). We booked our trip in March through one of the online services (not sure I can mention the name). If you'd like to explore anything other than these resorts, it's going to need to be booked 11 months out as this is when bookings open for members at their home resort. Bookings for non-home resorts start at 7 months out. I checked for my co-worker who is considering going this September and 45 days out there was still availability at OKW, AKL Kidani, and SSR.
DVC Rental is a bargain of you're okay with the following things:
1. Much more restrictive cancellation policies and larger deposits. While the service we used allowed final payment at 45 days out, many DVC rentals require full payment. There is also pretty much no cancellation - book only if you know that's the dates you truly want.
2. No daily Mousekeeping. You get trash and towel service on day 4 and a full clean on day 8.
3. You don't have full control of your reservation. If you want the Dining Plan or DME from the Airport, the renting DVC member has to add these items for you (or if you go through an agency, they will coordinate for you).
I'm willing to give these things up to stay at a deluxe for less than a moderate would cost me in September.
You do pay a $2-3 point premium by going through an agency, but I like the convenience and peace of mind which is why I'm willing to pay the premium. If you want to save a few dollars, there are plenty of reputable and honest members here on the Dis that rent out their points. Check out the DVC board.