I worked for a company that provided (private) hosted contact centre solutions and ended up getting into providing MS Teams Calling.
One DC in Melbourne AU, one DC in Sydney AU.
We needed to be able to fail over between Melbourne and Sydney and handle traffic on multiple ISPs and on IXs (IXs are small communities that ISPs create amongst themselves to send traffic between each other at little/no cost).
We created an account with APNIC, requested a /24 IP block (you can get less than 255 IPv4 addresses - a /24 block), paid about $1,500AUD and got setup (they gave us an ASN and an IPv6 block too)
That cost is each year.
You then go to an ISP and tell them to use your IP addresses and ASN.
You might need to peer (as others are saying), or they might just send the whole IP range to your firewall and you do whatever you want with it.
One thing to be aware of: if you start using more than one ISP in different locations, you may receive traffic from ISP1 and return it back via ISP2.. Be aware of this when dealing with firewalls (either your firewalls or your customers)