Get an old style Dynatec [sic]
/thread. Just go Dynate
x for timed. For impact I'd normally say TRMR, but it looks like they really are out of production and stock now.

There are other brands of impact grenades including some that take 3 or even 7 blanks, but they tend to be single-batch manufacturing - only buy from stock, never on a promise. And in particular don't give your money to the rogues at SWAT as you'll be looking at months before you see what you've paid for.
For example (other retailers are available!)
https://www.defconairsoft.co.uk/product/dynatex-timed-delay-grenade-bfg/
https://www.defconairsoft.co.uk/product/dynatex-dominator-impact-grenade-bfg-100-blanks/
Gas BB-shower grenades are a bit feeble, I've seen them go off at players' feet (self included) and they haven't even noticed either the sound or the gentle patter of BBs (if any even hit them). If you need to shout "BANG, YOU'RE DEAD!" you might as well be throwing dummies.
The gas grenades that make a bang by popping a plastic window are also erratic, I've heard them go off fairly loudly, but also make just a sad little "pop".
Some grenades can take either .209 primers or bigger 9mm. 9mm is much louder, but 0.209 is much cheaper. .209 primers make a satisfying bang indoors. In noisy woodland they can be missed, but 9mm blanks cost so much more that I'd rather fling .209 and shout "BANG!" (nobody can accuse me of being consistent

)
Disposable, no thanks. As shark says, the number of times I've seen folk just take cover, or even kick them back (I know, don't kick them back) makes them very poor value for money. I actually make a point of standing still and taking the kill just so that the player that threw it gets their money's worth. But when you add up how much they cost, you quickly get into the region of buying a Dynatex. The big difference is that with a blank-firing grenade you're paying pennies per bang,
and you have an asset to sell.