I read that familiars can be useful for communicating with spiritual entities. How do I get one just for the time being till I can do it myself?
If you want to get a familiar only "for the time being" to assist you with communication, until you can do it yourself, you're wanting one for the wrong reason.
To use a literal soul as a puppet, to use it for your benefit and when you don't need it, you don't want it anymore, is not ethical in any way, shape or form.
Knowing that, the Gods most likely wouldn't communicate with you through a familiar, to not make you dependent on it. This if they gift you a familiar in the first place.
The Gods always have ways to reach out to you when they want you to know something. You just need to pay attention to any signs. This being said, you are always advanced enough to receive messages from them, that's because, if they really want you to know something, they Will make sure the message got to you, regardless if you hear their voices or not.
A familiar is a huge responsibility, only gifted by the Gods to those who are suitable for a long-term companion, throughout their advancement journey. If you want a familiar for that purpose, do a ritual to ask the Gods for one. Considering these are actual souls with emotions, if you want one temporarily, better rethink your choices.