I’ve been lucky with Vivanuncios: #1 in Bienes Raíces en Mexico on occasion.
You can also ask Spanish Magic (language school north of Santo Domingo) as the owner has lots of local information and runs a fantastic Spanish school. At one point, she was coordinating intercambio sessions at the Oaxaca Lending Library (Oaxacans with English speakers – it’s great fun). She would therefore surely hear about apartments for rent from her students or intercambio participants.
Also, if you walk to the Oaxaca Lending Library (Pino Suarez), they had a book of apartments available.

If you plan to live in San Felipe there are bungalows, some on Primero de Mayo and some on the main street leading into SF on the right hand side (I believe near the church but not sure). You can ask either place. The ones in Primero de Mayo are nice – I stayed there for about 5 months.
There are apartments right in front of Sto. Domingo Church – behind a large wooden door. Hard to find, not in great condition but fantastic location. I know there is a resort on the southeast side of town and they were recently listed on vivanuncios. They look like very nice apartments.
It’s my personal opinion that I don’t feel good negotiating with people in Mexico or Guatemala. I feel like the small amount I could save doesn’t mean much to me, but could mean everything to them. Did I get the ** price sometimes? I’m sure it did, but since I can’t even remember specific times, it didn’t really affect me. I’m sure if your husband gets in touch the price will be right.
Hope this helps you.