It depends a bit on how much you want to spend, whether or not you're comfortable with something that doesn't have corporate support.
The Stratux is an open source ADS-B IN device that can either be purchased through iFly https://www.iflygps.com/STORE/Product/stratux , or from several other sources that will sell a similar product packaged their own unique way and are ready to use upon delivery.
You can also purchase all the components of the Stratux either on Amazon, or a combination of Amazon, ebay, and/or the local electronics store. You will have to put it together yourself.
iFly also sells the Astro Link https://www.iflygps.com/STORE/Product/astro-link-ahrs-gps-ads-b-receiver a ready to use portable device that seems to be not a Stratux clone.
I have a Stratus (not Stratux) 3i that was installed in our plane along with the Stratus ADS-B OUT transponder, as we felt we needed a new transponder anyway.
There are a number of other options that I know even less about than these.
I originally was using a Stratux and had no problems with it. The down side of my cobbled together Stratux, (and all the Stratux versions), was that it needed a battery to power it, had to be placed in a location that both could get radio reception , be stable for the AHRS, and not interfere with anything in the plane. I had it sitting on top of the dashboard. Worked fine, but I only flew in smooth VFR weather. I don't know how long it would have stayed there in turbulence. I was going to fashion a mount to keep it relatively secure, but we got the Stratus 3i installed before that happened.
The Stratus 3i is powered by the aircraft power and mounted under the dash, out of the way, no wires, no setting it up before each flight, etc. I don't see any practical difference between it and my old Stratux when I look at my 740.
Stratus does make a purely portable Stratus 3 that is the same as my 3i, except the 3i doesn't have a built in battery. If you purchase used, be aware that any Stratus numbered less than 3 will not work with iFly. The earlier versions only worked with ForeFlight.