What to buy is real real real (reel) tricky. Need to know how many devices your are using on each network, what kind of network (ethernet, fast ethernet, gigabit ethernet?) How much communication between networks there is. Depending on what you require between them you could spend as little as $50 or as much as $5000.
Ok we will have to start with some assumptions. And we will be working with the simplest solution, the most basic config possible. More info on what you have currently would be needed to go farther.
You have 5 people on each network that do not need to communicate much or often.
You have 2 switches, one for each network.
You have or will have a 2600 series or better router with 2 Ethernet ports.
You are only running ethernet (10baseT)
Connect each Ethernet interface on the router to the switch for each network, using a plain old ethernet cable. Interface e/0 to the switch running the 192.168.1.0 network, and e/1 to the 192.168.2.0 switch. Once connected you need to configure the router interfaces. (example below) Once the configuration is complete you will need to change the default gateway on each network to point to the ip address of the newly added router interface. So on 192.168.1.0 those machines need to have there default gateway set to 192.168.1.254, and the other network needs its machines default gateway set to 192.168.2.254. Once thats done they should be able to communicate.
Enter configuration commands, one per line. End with CNTL/Z.