I don’t know if you fish, or like to walk the marshes or shores in the area, but if you do, I have two great options:
Grass shrimp.
1. Collect yourself (free)- where: Any marsh with freshwater input on the ocean.
Look in the flow of freshwater into the high marsh, in narrow streams, that are overgrown with filamentous algae. The grass shrimp spawn in spring in the low salinity waters along the upper marsh. Scoop the algae clumps with a dipnet. The shrimp are translucent to brown or green, depending on the backdrop. They jump around so grab them out of the net and put them in your bucket quick.
2. Fore River Bait and Tackle in Weymouth has these for bait for smelt fishing. Maybe not this time of year, but if you are near them they might tell you where they collect locally.
Smaller individuals may suit your lionfish better if it is small, but they don’t get huge. Think peppermint shrimp size.
Storing a number live is easy in a cool spot with airpump in your house. A five gallon bucket could sustain a two week supply pretty easy.