So many options for you! Looking for new or used? I know my headsail was trash when I got the boat (took the sail to a recommended loft, and he gave me the bad news), and since I didn't want to spend the money for a new headsail - plenty of other stuff to fix - I got a used sail to get me out on the water for about $100. At least that way, I wouldn't cry if the boat turned out to be a bad purchase!
Anyway, here are some choices:
New Sails - Bluewater Yachts -
http://shop.bwyachts.com/category-s/352.htm
New Sails - Sail Warehouse -
http://thesailwarehouse.com/cgi-bin/web_store.cgi?page=storeframe.html&cart_id=
New Sails - Pe ak Sails -
https://www.peaksailsna.com/macgregor/macgregor-25/
Used Sails - Masthead Sails -
https://mastheadsailinggear.com/used-sails/
By the way, the guys at Masthead are GREAT to work with. I actually called them with my sail measurements, and he went thru his inventory and found me a sail that would get me out on the water. Very knowledgeable and friendly.
Speaking of the measurements for my sails, the only way I knew what they were was because I took a drive out to Tampa with my sail to the loft because I wanted both an evaluation, and a bit of education about my sails (first boat), and he was kind enough to give me the measurements of the sails, so I sounded half intelligent when I was talking to people. Some of the sites you needed to the numbers to search for sails, so if you don't know what size you're looking for, let me know, and I can post the numbers for my sails.
Good luck, and happy shopping!!!
-Peter