Can't claim to be an expert on the topic myself - there is a
subforum devoted to the topic, though, which may give you a few ideas.
A couple of suggestions to think about;
- What skills do you have available? Any skill that produces something (Painting, Writing, etc.) will give you a source of income - my current painter after reaching level 10, completing the challenges, and getting the Extra Creative reward regularly paints up Large pictures worth 3-5k, for instance, and can do at least 2 a day if it's her main focus - and if you're sticking with the challenge you should at least have a mid-level painter, someone working on gardening, and a fisherman/woman, all of which can make a fair profit.
As far as I know money tree seeds don't have a specific spawn point; first game I played I had the opposite problem, I wanted life fruit but all I got was money trees! The best suggestion I can make is that more is better; if you find and plant 20 special seeds you'll get more chances than if you plant 2, so check the graveyard, science facility, and other spawn points that I've forgotten on a daily basis.
Oh, and if you've been working on a Sculptor, if they
don't have the Savvy Sculptor benefits, they will break sculptures intermittently - which, in the case of Stone, can reveal high-value gems and metals (including Supernovium)
- What - if any - careers do you have your Sims following? Career paths in my experience are a little odd, because to start out they give little benefit for the time invested. However, at the top of a career, you'll start to see better results - especially if you take a top level career for an immortal Sim who'll be around for a long, long time.
Self-employed professions (ie. the skill-based ones) don't give any money themselves, but you do get bonuses at certain career levels - of course, how valuable a bonus of 5k is when you have to sell ~90k worth of stuff to get it is questionable.
It's kind of hard to define a
best career, as you've also got to balance other concerns such as working hours and additional benefits (the Culinary career for example is fairly low pay, but gives the special fridge at the top career level).
Reading the thread I see you're in the Athletic career - the base pay is sort of middling, but the real prize for that one is when you start playing (and winning!) games, as those give more money than the base job normally does. Get your Athletic skill to the top and use the 'Prepare for Game' work tone on your 'normal' working days when games become available, and you should find yourself earning a fairly steady income from this alone.
- Collecting is always an option to raise some quick cash - unless you know where spawn points are or use a map, grabbing the Collection Helper reward with someone is a good idea for this. The base neighborhood doesn't have much in the way of ultra-expensive items (except for Pink Diamonds, which are worth over 1k uncut, but are very rare), but you can still make a bit from selling on lots of stuff. If you've got World Adventures then you can also play around with Transfiguring, which can net you quite the sum of money if you play it right.
- If you're really desperate for some quick cash and have been tending to your Sims' wishes, anyone who has spare Lifetime Happiness points can trade them in for some cash (non-immortals preferred as otherwise this reduces the available 'unique rewards'). Any reward that results in an object (eg. Collection Helper, Hover Bed, Teleportation Pad) can be sold, yielding about 20% of the Happiness spent as cash; if you've got Generations then you can use Inheritance for a better return. This is obviously only a short-term solution, however, but if you're pushing the end of Elder stage and are a few thousand short of that last property, this can save that generation.
Don't forget that the only time limit (besides death, of course) is on museum pieces. If you are part-way through Adult, you've still got the rest of the Adult phase to go, plus the initial phase of Elder, before you're at risk - if you've earned §30,000 already, then chances are you'll earn that much again in the same period of time as a minimum, which gives you enough to buy the cheapest building (Diner type properties are priced at 24k) and should let you place a few bits and pieces if you need to upgrade something.
