I have just completed a border bounce to Cambodia and back with a 30 day entry stamp. Is it possible to get a 30 day extension near the end of this one at an immigration office in thailand?

Yes, same as with any other visa exempt entry. 

OP, normally apply couple of weeks prior to expiry of 30 day stamp.

If you apply in Bangkok you would attend Laksi immigration office. 

Removed an off-topic post.

On 5/4/2023 at 6:09 PM, DrJack54 said:

OP, normally apply couple of weeks prior to expiry of 30 day stamp.

If you apply in Bangkok you would attend Laksi immigration office. 

At most Imm Offices you can only apply for the 30-day extension of stay from 14 days before to last day of your VisaExempt acquired Permit to stay. 

No need to wait till final days to do it as the 30 days will be added to your Permit to stay expiry date.