Start with your local shops - some may have trips planned for September that you can join - people cancel at the last minute regularly. So you'd get to meet the other divers/potential buddy before you go - usually there's some sort of social/go over the last minute details meeting.
Another option is the singledivers.com group is doing a Singles trip to Bonaire on the 5th if you can work that out by then.
https://singledivers.com/home/bonaire-2015 This is a true singles week so you should be able to find a buddy fairly easy. It looks like they're staying at the Divi so you'll meet other divers on your boat that you can then shore dive with in the afternoons.
Diving on Bonaire mostly couldn't be easier - the reef parallels the shore, the water is crystal clear - so much that you usually see the mooring ball/rope when returning to your entry point. Bonaire has no rivers and the whole island has been a Marine Park since the 70's and there's little/no agriculture so no run-off. Makes for some really good reefs which promote a lot of fishlife. Bari Reef there - an easy shore dive - has the highest number of counted species in the Caribbean. That's partly because Bonaire Dive/Adventure encourages divers to do REEF fish counts with their on-site naturalists but it's still a lot of fish. My buddy counted 185 species in a 70 min. dive.
All of the dive sites are marked on the surface by yellow painted rocks and many divers have piled up coral rubble indicating the best entry point. Check with singledivers, they likely made provisions for two or more to share a dive truck - they're 4 door pickups so most people won't mind a tagalong if asked.
Downside is that the entries are rougher. Most are ironshore on shore, some ironshore off shore and most have coral rubble in the surf line. So good
treaded boots are a must if you don't want to go down. You walk/crawl/float out to deeper water and put your fins on to dive. Ironshore is also sharp and gloves are not allowed. See the site entry pictures here for examples:
Scuba Shore Diving Region: ABC Islands
Someone recently posted
Sunset House on Grand Cayman is offering a discount on single diver rates also on their garden view rooms packages. You get all the boat dives and room/breakfast at that price. My Bar there has good food, there's the Sea Harvest restaurant and downtown Georgetown is walking distance. Once the cruise ships leave it's very pleasant and there's a few decent waterfront restaurants.
If you can't find a buddy there for afternoon shore diving you're really not trying. Everybody in the bar is a diver/knows one and the day we were there at least a dozen groups walked past us to their dive dock during lunch. There's a couple of 60ish foot dives there as well. The only downside will be that many people on the morning boats are looking for a 100' wall dive first so that may be difficult for you to work out - the moorings are at 60'. Every dive has at least one DM in the water so it is possible. You'll be able to do all the 2nd dives as they're often in the 40-60' range. The signature dive there - Stingray City is 14' deep. Sunset Divers does it as as afternoon 3rd dive. The other is the Kittiwake wreck - it almost breaks the surface. The first 3 levels are penetrable by anyone with OW. Sunset should have multiple time slots there.
hth,