is there a matching monitor i can buy to match my iMac so i can have two monitors?

There are places you can get "Thunderbolt Display" - iMac and this display would be only connected with single thunderbolt wire (of course, you will need Thunderbolt 3 -> Thunderbolt 2 adapter). I used this set up before, it would gain 3 USB 2.0 ports, 1 FW 800, 1 Thunderbolt 2 and 1 ethernet port, with extra bass sound from this diplay.

https://support.apple.com/kb/sp642?locale=en_US

It is 27" and fits well side by side with my 27" 5K iMac -- if the resolution 2560x1440 fits your need.

The only external display Apple makes is the Pro Display XDR which is designed for the Mac Pro. However there are hundreds of external displays you can select from that will work fine with your iMac. You have to search for the resolution the iMac supports on the external display and then locate external display that match that resolution and look for the size. There is no "best" you have to do your own due diligence.

Apple once sold displays that looked like modern iMacs. In fact, they looked so much like an iMac that there was a post here about a year ago from someone who bought a "used iMac." They could not get it to boot. Turned out the poster had bought a 27" Cinema Display.

There are some used and/or refurbished sample on the open market but are still very pricy. See this for how they looked and how much of a wound they could cause one's wallet:

https://www.amazon.com/s?k=apple+cinema+display&rh=n%3A1292115011%2Cp_89%3AApple&dc&crid=1HWMERA9I8KQQ&qid=1597938341&rnid=2528832011&sprefix=Apple+cinema%2Caps%2C228&ref=sr_nr_p_89_1