[Android Central Question]

I was told the phone was not compatible because of frequency.Where do I buy a sim card that will work on my unlocked phone without buying a contract?

[belodion]

For what network? You’ll have to choose a network whose frequencies are supported by the phone, and which offers PAYG.

[Rukbat]

As belodion said, you have to do it the other way - find a carrier that uses the freqwuencies that your phone supports. If it's the K6000, go to https://www.devicespecifications.com/en/model/bebd3771 and scroll down to Networks. Then find a carrier in your area that uses those LTE (they may call them 4G) frequencies. The 2G frequencies are universal, so you'll get GSM voice and text all over. (But not CDMA - if any carriers near you are CDMA carriers, don't even bother looking at their frequencies.) But you'll need compatible LTE frequencies to get data and MMS (attachments to texts).