Would I be allowed to get a haircut today (Sunday) since the lady who cuts my hair usually isn’t available on Fridays?
Yes, you are allowed to take a haircut on Sunday’s etc. even though it says that it is kavod Shabbos to take a haircut close to Shabbos (Friday), nevertheless it is still permitted to take them on other days of the week. On should avoid taking a haircut on Thursday if possible. The reason being because the hairs regrowth starts becoming noticeable 48 hours after they are cut, and it wouldn’t be kavod Shabbos to notice then starting to grow back. Even this is only a preference, and if it can’t be done on Friday, then it may be done on Thursday.