Welcome to karting, sorry about the Maxjet, they are probably out of business like you suspect, the 2008 downturn was pretty tough on a lot of karting related businesses.
As far as tuning goes you already have the most important tool for getting a 2-stroke mixture right (as long as your ears are working). Try to get with some of the other Rotax guys at your track, they should be willing to share good starting set-ups and teach you how to tune from there. Remember, the most important tuning tool you have is the stopwatch ;)
There are other density/altitude gauges out there if you want to get more technical but until you’re fighting for that extra 10th that you can’t get any other way (ie. driving) you probably won’t need to be changing jets in the middle of the day.
If you’re curious about how the weather effects your engine you can get density/altitude numbers from your nearest airport, pilots use them to calculate fuel loads it think….
Finally, if there’s someone at your track that does driver coaching consider hiring them for a few sessions. They’re usually knowledgeable about engine and kart set-up and can get you started in the right direction (and keep you from buying stuff that won’t help). As a new driver there are few products you can buy besides tires and fuel that will make you any faster, it’s all about seat time.