My recommendation is to find an apartment to share or rent alone (depending on your finances) in one of the following locations and to pick a beis midrash as there are usually contacts who can set up chavrusos. Some of these might have options for shiurim you can attend.
Ordered by likely best fit I would suggest:
- YU/RIETS in Washington Heights - for years they have catered to singles who are former students (and sometimes others) who are not officially studying there but want to live in proximity to a yeshiva environment. You also have the option of local eateries and can possibly join their student Shabbos meals by paying as a guest (if someone knows that the current situation is different then please correct me). They have Kollel Yom Rishon which is geared toward non-students. It might not be the best option re: learning a consistent masechta, but if you contact someone there or at RIETS they might know of ideas for learning programs that would be a fit for you. It's still in Manhattan so you have the advantage of the subway if you work in the city. If you own a car, word to the wise to park it in a safe place (but I guess that goes for many places in NYC). Not the greatest neighborhood, but since you are coming from a Modern Orthodox background this might be the best cultural fit for you.
- Kew Gardens Hills in Queens is commutable to Manhattan and there are a number of yeshiva options there where you might connect. Ohr Hachaim comes to mind and it's common to see outside neighborhood people learning there. Beis Medrash L'Talmud (aka Landers) and Chofetz Chaim are also there and depending on where you find an apartment they might be more convenient (but I'm not aware on a first-hand basis how common it is for outsiders to learn there and whether they offer programs catering specifically to non-students).
- There are other communities with working single professionals that are commutable to Manhattan which are a bit more yeshivish but you might still find your niche. Passaic, NJ comes to mind and you can check out their Passaic Torah Institute and Passaic Clifton Community Kollel. Of course there is Brooklyn to but I'm not as familiar with the options that would cater to your situation.
If you have a decent personal network in any of these locations, it can affect the speed in which you set up your apartment and chavrusa. For some of these options, security has become a more serious issue over the years, so it would be advisable to inquire as to how visitors can gain building access for the beis midrash.
Much hatzlocha with your efforts!
- I have no official connection with any of these institutions.
- The above ideas are relevant to the present and might not be pertinent if someone views this question years from now.