Reichstag - can't find tickets

Unfortunately, no. It is a government building so they have security measures in place; one of which is that you must book in advance. I would keep trying—it's always possible something will open up, but you won't be able to just show up and go in.

I remember reading on here some time back (don't remember how far) that someone was able to go somewhere and see if there were any tickets available for that day or the next.

I found this on their website: "Information for visitors who have not booked in advance
If you would like to visit the dome but have not booked in advance, you can register to do so at the service centre run by the Visitors’ Service near the Reichstag Building, next to the Berlin Pavilion on the south side of Scheidemannstraße. If any free places are still available, you will be issued a booking confirmation entitling you to visit the dome; please note that this must be issued a minimum of two hours before the time of your visit.

Important: To book your visit to the dome, you will need to provide the following information at the service centre: your last name, first name and date of birth. When registering in person at the service centre, every visitor (aged 16 or over) must show a valid official photo ID (identity card or passport). Young people aged 14 or 15 should show an identification document (student ID card, passport or similar document with their name and a picture on it). Please ensure you bring the original, as copies of identification documents are not accepted.

The booking confirmation is issued to you personally and is non-transferable. You will be asked for proof of identity both upon registration and at the main entrance for visitors.

The service centre is open during the winter month (November 1 to March 31) from 8.00 to 18.00 hrs and in the summer months (April 1 to 31 October) from 8.00 to 20.00 hrs.

Attention: On weekends, the service centre is only open from 10.00 to 18.00 hrs. In addition, please note that the service centre closes at 14.00 hrs on Tuesdays during session weeks."

I beleive that information is up to date.

Website: https://www.bundestag.de/en/visittheBundestag/dome/registration-245686

Read part "Spontaneous visit to the Reichstag dome".

Details about closures: https://www.bundestag.de/en/visittheBundestag/dome/registration-245686

Thank you Mardee, Nancy & MarkK.
We will reach tomorrow -- I'm hoping for the best.

Update:
We went to Visitors’ Service Center today in person, and were able to find tickets for tomorrow. We were able to do this with our CA drivers license as IDs .. However, we will need to carry out passports when we visit tomorrow. While the online site did not show any openings until the 29th, there were many available for tomorrow and day after. You just have to go there in person.

Thanks again for your help - much appreciated.

Hope, this should help others.