nyc.bestparking.com is great. If you type in an address and time frame, it will give you a map of garages in the area, along with a rate for each. (You sometimes need to click on the garage you want in order to have a coupon mailed to you with rate/reservation). I'm sure there are other apps as well.
For future reference, you can find free street parking after 6 in Tribeca and much of Soho. I'm not sure about other neighborhoods. I will often park there, then take a subway to where I need to go.
Street parking in Midtown is challenging... if, next time, you can schedule a date for after 8, though, you'll be able to find spots on 10th Ave. and some side streets. If it's after 10, you can almost always find a spot. Some are paid (metered), some free. The metered spots will generally run you less than $10.
If you can arrive a bit earlier, try Riverside Drive and UWS side streets (70's through lower 100's). Especially if it's a weekend, or if you can sneak in at the tail end of street cleaning rules. (Generally Mon/Thurs or Tues/Fri 9:30-11, or 11-12:30).