Budget building with room for upgrades

Elaras

Any low end tweeter should due until you buy a new set.
I would go with something cheap like this.
Pyle PLST6 1/2" Neodymium Tweeter Pair
They may not sound awesome. But will be better then no tweeter.
I have installed low end tweeters like this. Most times they are loud and harsh.
What I have done in the past is to buy 2 sets and run them at 8 ohms. Or you can build a custom crossover.
You can also point the tweeter away from your ear.
Most times you can make them sound ok
I have never used the tweeters I linked, just for reference.