Completely hiding lavs & transmitters without having a lot of extraneous noise such as clothing rustle, etc. takes a bit of skill.
Hiding lav/trans systems are easier when they are smaller. There are some
very small transmitter units which are, however, quite pricey. The less you spend the larger everything will get and, as you get into the cheap zone, limited range and reliability.
Lectrosonics makes a transmitter about the size of a matchbox.
It's about $1,500.
Quality lavs are equally pricey.
You can get consumer wireless lav systems for under $100. The RODE GO system is about $200 without a lav.
I did a series of audio clean-up jobs for a client who would mount a lav under the brim of a baseball cap so it was pointed at the person to whom he was talking in addition to the lav he wore himself.
IndieTalk is talking about a parabolic mic dish with a shotgun for long-distance audio recording. You may have seen some folks using these at sports events and field recording nature sounds.
An option to think about if you have someone to wield it for you.
Give us a budget and we'll cram as much as we can into it.