Suzy,
Do I understand correctly that the existing photo is 24x30? No consumer flat-bed scanners that I am aware of are that large. This leaves you with several options.
1) Use a smaller scanner to scan the photo by sectors, then use 'stitching' software to put the pieces back together into a single scanned image. Then, as R.J. says, put the resulting file on a CD-ROM and take it to a printing shop.
2) Find a pro shop that has commercial scanners of that size. They may not be cheap, but I have no doubt that they can do the job. As a one-off job for a person, they may be able to cut you a break on the cost if you ask.
I hope you find this helpful, and welcome to RFF.
Best Regards,
Bill Mattocks