Initially seahorse fry will only eat live foods. Don't even try frozen foods for the first few weeks - you will do nothing but destroy your water quality which, in turn, will cause the fry to suffer/die off.
Once the fry are eating well and you've gotten them to be a few weeks old, then you can consider introducing some frozen foods to see how it goes. Fry must be "tricked" (or "trained") to eat frozen foods. You can't just drop it in the tank and expect them to eat it. I can explain it more when you're ready for that....but we can cross that bridge in a few weeks.
Rotifers are much smaller than baby brine shrimp. They are so tiny you can barely see them in the water. I cultured them for my northern erectus fry as well as my pipefish fry. I can't recall seeing rotifers for sale at any LFS around me. Maybe you'll have better luck in your area. Usually, to start a culture of rotifers I order them online and then culture them myself in a seperate tank.
You will need to at least find someone near you (local reef club?) that already has rotifers culturing or buy some to culture yourself.
Raising seahorse fry is not easy or simple. WHen I had northern erectus, I had two 10 gal tanks of rotifers, 3 brine hatchers and the tank of fry. It can get rather complex and time consuming, so don't be discouraged if a "shot-gun" approach doesn't yield good results. Quite honestly, if you weren't prepared ahead of time for fry, the outlook isn't too good.
As far as feeding, I feed newborn fry 3 to 4 times daily and siphon the bottom of the tank to remove poop and debris AT LEAST twice daily. This helps with water quality and reduces any likely ciliate issues within the fry tank. I use two pre-cycled large Hydro sponge filters in the fry tank to keep ammonia in check. The biggest thing with fry is to feed often and to keep their tank rediculously spotlessly immaculately clean. (Did I stress enough how important it is to keep the tank spotless? LOL )
I can't send you rotifers as I no longer keep northern erectus. I only keep/breed Southern Erectus and dwarf seahorses presently. I can give you places to buy rotifers if you PM me, but shipping overnight isn't cheap and with the holidays coming up I"m not sure if you can get them in time.
Best of luck..
Tom