I don't know about dedicated app or a website but I was using Binance for that, cons is that it can't automatically alert automatically the way you wish:
So if the price is decreasing, and I've set it at -%1 , once the price hits $99, I'll be altered, and then alerted again when it hits $98..and so on ($97, $96, $95...etc).
Okay, in Binance, you have to move on
Market's section, choose the coin you want to track and then click on
Alert. Then there will come a popup of options that incllude:
1. Price rises above (you have to enter the value and alert frequency).
2. Price drops to (you have to enter the value and alert frequency).
3. Change is over (you have to enter the percentage and alert frequency).
4. Change is under (you have to enter the percentage and alert frequency).
5. 24H change is over (you have to enter the percentage and alert frequency).
6. 24H change is down (you have to enter the percentage and alert frequency).
Btw if you need alerts so frequently, what about to just spend more time in viewing the carts? Cryptocurrency prices change so frequently that it will really mess with your attention.