Hi Maria!
For the most part, it's a good idea to purchase
Disneyland Resort Park tickets online.
To answer your question–– yes, there is a possibility that if you buy them in person when you arrive, the ticket booth prices may be higher than what you had seen online.
That's because the Disneyland Resort has a policy where they may adjust pricing without advance notice, depending on several factors. One of those factors is whether the Parks are really crowded that day.
The busier the Parks, the more demand there seems to be and
Park tickets tend to increase.
Nonetheless, if you happen to visit during a high peak time, and you're staying at one of the three Disneyland Resort Hotels, you can take advantage of several hotel guest perks, including the
Extra Magic Hour, where hotel guests are allowed to enter the Parks one hour before regular Park opening.
You can also consider visiting between mid-January and mid-February when the crowds tend to reduce and then again in the Fall, between mid-September and mid-October.
I hope this helps!
Thanks for visiting the Disney Parks Moms Panel!
Have a Magical Time!
-Anabel M.