patricej

only use singer bobbins in singer machines.
those are class 15/J.
beware of generic bobbins that claim to work in both A and J.

the metal bobbins with holes will do "ok" in place of a drop-in 15/A but they are not ideal.
those are what you need for a side-load bobbin in a metal case.

any brand class 15/A bobbin will work in a brother machine that uses class 15.
most newer brothers do, although i think some of the older models might use class L.
(don't quote me on that.)

i wouldn't buy bobbins from walmart.
(no ... i am not anti-walmart. i just think their bobbin classifcations are a bit "iffy.")

if you are near a hobby lobby, the sewology class 15 bobbins they sell should do nicely.

there are a bazillion places online to buy 15/A bobbins.
prices will vary wildly.

your biggest challenge will be finding some you can receive in just 2 days.