5.0.2 is the only compatible release with the mentioned CUCM release as you are saying.
UCCX media kits are not available for download, just service packs, since you will need to deploy the newer release from the start your only option is to order it via Product Upgrade Tool instead of downloading the Service Pack from CCO.
If you have a valid 5.0.x license and PAK you will be able to use them on the newer 5.0.2 system
Hope this address your question.
PS. Please remember to rate useful posts accordingly