You can buy Jtags anywhere. Ebay or search it up on google. Or other gaming sites.
I suggest buying them at TTG
Skills, well anybody can learn how. You just have to read guides on how to do things.
You buy KVs from getting banned and the lowest is $15 for a day.
What Jtag do I need to buy? Dont buy an Xenon. They CAN last for a while but you really cant use them for a long time. Very prone to RROD and overheating. I had a Falcon and I sometimes left it on for days and It stayed cool. Very very fast boot time too. RGH v1
Where should I buy it? I bought mine offsite and it was dirt cheap too. Not ebay or craigslist or anything like that.
What's the skill level on using a Jtag? Basic knowledge. Mostly common sense.
Where do I find mods? You 'find' mods either by RTE for newer Call of duty's (Real time editing) or by downloading patches and putting them in the right directory on your harddrive for older Call of duty's.