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You're welcome. I also forgot to mention census records. I read that those may be a little harder to obtain, but they also serve as proof of her physical presence. If your mom can help you obtain any of these docs, that would be great, since some things would be easier if she got them herself like tax returns.
Basically anything that can help prove her days in the US, any leases, bills she paid, etc. Are her parents still alive? For her parent's tax returns, your mother may be able to request from the IRS if they are deceased. I also read that affidavits from family members or friends help, they don't hold that much weight though, but they help support the rest of the evidence.
Talk with your mom and make a list of all the things she did in the US (school, work, arrests, traffic tickets, court dates, any kind of appointments, church, forms she filled out, etc) and anything she may have done as a child. This will help you figure out where else you can look for evidence. Good luck.

By the way, it's never too late to prove your US citizenship, it's just a matter of obtaining the documentation. There is no time limit to a claim of citizenship by birth.

Does your mom have any of her old passports? You may be able to use the visa stamps as one form of physical presence evidence. Also you can do an FOIA request with CBP for your mother's entries to the US and hopefully exits as well to determine the dates she was physically in the US. Partial days in the country count as well.
If she was in the US up until age 5, plus the 2 years in her 20's, she meets the physical presence requirements. You can probably use additional docs to prove her physical presence such as her parent's tax returns if they claimed her as a dependent. Doctor's records from when she was a toddler, school records, church records.
If she worked here she can request FOIA or PA records from the IRS or SSA if she no longer has access to her pay stubs or former employers. There are plenty of ways to prove her physical presence here, it's just a matter of how hard you look.
The reason I suggest that you try to establish your USC is because it would be much more convenient for you to enter the US as a USC than go through the painful, expensive, and tiresome process that is immigration in the US.
Birth Abroad to One Citizen and One Alien Parent in WedlockA child born abroad to one U.S. citizen parent and one alien parent acquires U.S. citizenship at birth under Section 301(g) of the INA provided the U.S. citizen parent was physically present in the United States or one of its outlying possessions for the time period required by the law applicable at the time of the child's birth. (For birth on or after November 14, 1986, a period of five years physical presence, two after the age of fourteen, is required. For birth between December 24, 1952 and November 13, 1986, a period of ten years, five after the age of fourteen, is required for physical presence in the United States or one of its outlying possessions to transmit U.S. citizenship to the child.) The U.S. citizen parent must be the genetic or the gestational parent and the legal parent of the child under local law at the time and place of the child’s birth to transmit U.S. citizenship.

Sorry -- not true. You can't get a Consular Report of Birth Abroad (CRBA), as they need to be issued before you are 18 years old. But, if you qualify for US citizenship, you are a US citizen whether the CRBA was applied for and issued. You have to meet the same requirements as you would for a CRBA; if you do, then you can apply for a US passport. Your mother would have had to meet two requirements in the years before you were born:
1. Been physically present in the US for at least five years before you were born -- that would include the time from her birth until she left, the time from when she returned in her 20's, and any visits.
2. At least two of the five years have to be after your mother was 14 years old -- the time in her 20's would count and any visits.