The best bang for your buck in getting a temp gauge is the one I used from B&M
http://www.summitracing.com/parts/BMM-80212/
This kit gives you everything you need to install the gauge and get it working. There are other gauges but most of them are just that, the gauge, then you have to get everything else to make it work.
As others have said a decent oil cooler of any kind, engine oil, power steering fluid, or transmission fluid, from a junk yard car will work and there are alot of nice ones out there.
If you want a new one then B&M has an answer there as well. I used this one
http://www.summitracing.com/parts/BMM-70268/
Comes with mounting brackets and hose to hook it up and get it working.
Now the B&M gauge I listed above has a mounting fixture that comes with it but I didn't like it and I wasn't a fan of the places other people had found to mount the gauge in a 97+ XJ so I bought this AutoMeter gauge pod
http://www.summitracing.com/parts/ATM-15211/
I posted a picture above of what it looks like mounted up and I really like it! its right where I need it to be to easily keep and eye on it, I didn't have to hack up my dash to mount it, and its different! By the time I was done I had close to $150 invested in this setup. Like others have said though, you can save some money getting a junk yard cooler and you don't have to buy the gauge pod so I think you could get this done for $70-$80 depending on what you pay for the junkyard cooler.