I remember reading on these forums that there's a small immigration branch office around Sukhumvit road where you can get a 30-day visa extension. Is that right, and where exactly is it? (I tried searching the forums, but my searches have been fruitless.)

I don't want to go all the way to Don Muang Airport. I just need a 30-day extension (1900baht). Thanks

No there is not. That is a special office for BOI One Stop Service and does not serve anyone but them. You must go to Chiang Watanna, Soi 7 for an extension of stay in Bangkok.