Knowledge is power, and not buying that screamingly cheap keg simply because it is not whatever style of connections you have can cost you in the long run. Depending on how well you shop around you can buy the pieces to convert a keg from pin to ball or vice versa for around $10. There are a few things you need to know.
What kind of keg do you have?
“Corny kegs” were manufactured by a lot of companies, with the lids generally being interchangeable, but the posts often use a different threading. You need to find the manufacturer so that you can buy the right threading. Look on the side of your keg to find inscriptions. Firestone and Cornelius are the two big manufacturers, so look for those first. Another useful tip is that firestone kegs generally have rubber that is not continuous. What I mean by that is that it is not one full circle at the top. It dips down in between handles. Here is a great link on how to tell different kegs apart.
What kind of fittings do you need?
This is as simple as it sounds. What does the top part need to be? Ball or pin? Unless you are terribly confused, and you ended up on this page by accident it’s probably the opposite of what you currently have.
Cornelius manufactured keg
Firestone manufactured keg
I ‘m willing to bet that 90%+ of people looking to do this are looking to convert a pin lock to a ball lock. There are simply more pin lock than ball lock kegs on the market right now, and they are selling for less. If you’re in the minority and you are looking to go the other way, make a post on one of the brewing forums and you have a very good chance of finding someone to swap with you for just the cost of postage!
Lastly, if you got this far down and you either decided it doesn’t make sense or is too confusing have no fear! There is another simple answer. Buy new fittings and make MFT your universal translator! If you are going to end up with more than a couple of kegs, then this can still be as economical as buying a lot of any one fitting. It also allows you to clean lines more easily AND start siphons more cleanly.