"Besides that, is it router must required to implement spanning tree??"
No, although with routers you can also define multiple paths through networks, though not using spanning tree. May not be any easier within your current network and would likely require readdressing many of your devices.
Depending on what your physical topology is, if it is one large ring, you might be able to insert just one spanning tree switch within the ring. This should block one side of the new switch unless there's a break in the ring on the other side.
If you have a star topology, two spanning tree switches could be used and connected to each other. Ideally with each existing switch having one link to each of the new spanning tree switches.
I.e., you might be able to provide spanning tree redundancy at the cost of just one or two new switches. (Note: such switches are likely to be more expensive than the those you've been purchasing.)
Other connection topologies are possible; too many variables to describe without seeing your existing topology.
As to how to configure, if you're not a network person, it might be advisable to contract with an external company or contractor with the experience for such configurations. It's not difficult, but if you have no experience performing network device configurations, having someone explain or show you the first time would help, especially since there are different variants of the spanning tree protocols and how they are activated often varies between vendors and sometimes even between vendor models.
