Decent speakers now a days cost a lot. You can explore the market and see what is available in which price bracket. I am also running new speakers without an amp and for a simple sound upgrade i feel my sound system is good enough without an amp.
If you have an Altis then it has 6 speakers (4 Main + 2 tweeters), if gli then 4 main speakers and if xli then 2 main speakers.
You can use the existing tweeter or get a complete component set for front which includes new tweeters as well.
For rear speakers that are 6x9 and is size mein ajkal speakers new a nai rahay so that will be slightly difficult to find. You could get a custom bracket made which converts 6x9 to 6.5 speakers out of wood for less than 500 or acrylic for 2000
If you want genuine original stuff not chinese items then go to sound valley in saddar.
I have cerwin vega with tweeters in my corolla running on stock grande pre-fl headunit.
If you live near DHA2 you are welcome to see quality or contact me on whatsapp if you need any help or guidance in installation hardware that is required