Are there immigration offices in Bangkok city center for the 90-day report?

You can take your passport to Bumrungrad Hospital on Sukhumvit Soi 3 at the 10th floor Business Center. They can do a 90 day report for you (doesn't matter if you're a patient at the hospital or not) for 650 baht

you leave the passport with them, they go do the 90 day report and then you come back to their office to pick it up

OR

You slog out to Chaengwattana to file a 90 day report on your own at the main immigration office in the government complex (Building B)

Here's the Bumrungrad Business Center link