At this moment, I have a SG300-28P switch, but it's running out of ports. I want to order another SG300-28p, but I have no idea what modules and cables I need to stack those switches.
Does anyone know this?
No much information in your question how you currently have the switch configured L2/L3 or both, but a quick answer is you aren't really stacking this model as it's not a stackable model. (stackables have only one IP address to manage the stack) You can daisy chain them together using one of the four ports on the front right side and configure as a trunk port on the existing and new switch. Can be technically done with any port but suggest one of the four. Or you can use one of the open SFP slots and buy two fiber modules.
Part number# MGBSX1 "1000BASE-SX SFP transceiver, for multimode fiber, 850 nm wavelength" and a 50 micron fiber patch cord between switches. Assuming in the same rack located together a 1-3 meter cable will do.
On most Cisco models if you use one of the SFP ports the the associated copper port is not usable.
All of the ports on the down stream switch are sharing the one uplink to the up stream switch. If this is a small office with mostly internet and low bandwidth traffic then this isn't a problem.
Again the ports are either configured as defaults and the two connecting ports between the switches will auto negotiate a link. But if an engineer has made changes to the default config then the link might not come up and spanning tree will shutdown the port.
Best to find a local network engineer that works with Cisco products and consult with them for the greatest success.
If you have some doubt about your bandwidth, you can configure between the 2 switchs an etherchannel, on the way to take a link with more capacity between A and B. If you dont know what it is an etherchannel, it´s a configuration that permit bound a few interfaces to make on logical interface. If you bound 2 Gigabit interfaces on the 2 swithces, you can create a link of 2 Gigabit between then, and so on.
Etherchannel will provides you more bandwidth, load-balance and redundancy.
You just buy another one and plug it in to the existing one with a Cat5/6 cable (or two for redundancy via an etherchannel). If they are far apart, get the miniGBIC for multimode fiber x2 (one for each switch, or x4 2 for each) and plug it in that way.
The Cisco 300 Series also offers mini gigabit interface converter (mini-GBIC) expansion slots that give you the option to add fiber-optic or Gigabit Ethernet uplink connectivity to the switch.