If you only need to detect IR pulses in a modulated signal then your best/cheapest approach would be to use ATTiny chips to activate after receiving say 'x' IR pulse within 'y' milli-seconds. The other parts were already covered here. Consider using button batteries, if size is an issue with AA or AAA.
If you need cheaper than the $9 digispark, then your own pcb with an ATTiny should work out better for 100 pcs - but more work. You may get a better price from digistump directly for 100 units?.
If you need to decode specific IR signal codes or shooters (like miles tag) then your software on the Attiny will become more complex. I supect a search would uncover a library that runs IRremote on ATtiny. When selecting IR receivers Vishay are good, avoid the cheaper ones (<$1)
PS: also consider using SMD IR receivers for an even more compact solution.