Edit: Melpomene beat me to it, and with a more helpful answer too. Ah well. But in case going through Etsy doesn’t work out, you have another option:

Secondhand BuzzBee shells can be found on ebay, as can unopened refill packs (with darts) - although for the latter at prices that’d easily make your ammo more expensive than your blaster. AFAIK this ammo system has been discontinued by the manufacturer.

The darts that this blaster uses are different from what has since become the industry-standard full-length dart: these ones have larger holes and the shells contain correspondingly larger dart pegs. So unfortunately you can’t just grab some shells and shove normal darts into them.

However - if you remove the dart pegs, that might work. These darts are also shorter than normal darts so you may also need to chop a little bit of foam off the back of each dart.

There’s also an untested 3d printable option for shells, which AFAIK should be compatible with normal darts as it has not dart peg.

This isn’t a blaster that’s widely used (most nerfers find the shells too easy to loose, especially in public games) so there’s not much of a knowledge base built up around it. You might need to do a little experimentation. I’d recommend starting with secondhand (or printed, if that’s a possibility) shells, removing the dart pegs, and using (if necessary shortened) normal darts.

For darts - Adventure Force darts are pretty good and can be bought cheaply in bulk at Walmart. I’m talking about the green darts with the blue heads. People in the hobby refer to them as “waffle” darts due to the tip design.

(Also, if you don’t mind a nitpick - the preferred term is “blaster,” not gun. This is for reasons which boil down to (a) avoiding dangerous misunderstandings and (b) setting an example that helps with (a)).