Gale

Super cheap: KZ Gale (~$5 but you'd probably want to add the QKZ T1 cable on for another $4-ish). Better build quality: EPZ Q1 Pro (~$35) or Tin C2 (~$30). Neither is strictly speaking neutral, but neutral IEMs don't exist. These are "neutral-ish" and tuned quite similarly with a balanced, slightly brighter tilt (which is what you need for soundstage & imaging). They all have good instrument separation as well (which is different to imaging). The C2 probably has the best separation & imaging, whereas the Q1 Pro probably has the widest soundstage. The C2 is fairly tip-sensitive and probably the brightest out of the box, with the stock tips, but you can tone that down if you're prepared to experiment a bit with aftermarket tips. The Q1 Pro is probably a bit safer otherwise.

Stick to 1DDs (single dynamic driver) to run off old hardware, because they won't change frequency response if the headphone amp isn't great, and while Apple stuff is usually better, older stuff isn't always that great. For around $5 on sale you can get the KZ EDC Pro, although recently I have been preferring the KZ Gale (the EDC Pro is bassier, the Gale sounds more resolving). Both come with crappy cables so you'd probably want to upgrade that. The QKZ T1 is good and cheap. Fits the Gale but not the EDC Pro. For the EDC Pro, basically any KBear cable (8-core or 16-core, the cheapy 4-core ones are passable, but nowhere near as good) with a standard 2-pin connector. There's also the incredibly over-hyped GK Kunten (also made by KZ) but while people were getting those on sale for around $2 (and that's why there's so much hype) the price has gone back up to normal now, so whether or not you can find one under $10 will depend what regional offers are running. I'd say they're perform about the same level as the Gale, so look for whichever one is cheaper or you prefer in terms of looks. The tuning is slightly different, but they're nearly identical. Both are QDC so both will work with the QKZ T1 cable. There's also the KZ ZVX Pro which I haven't heard myself, but apparently is the exactly same IEM as the Kunten (same driver, same tuning), just in a slightly higher quality all-metal shell, and now that the initial sales have passed, it's usually cheaper. So that's another potentially good option. QKZ T1 cable again. Unfortunately under $10 pretty much all your options are made by KZ, who are a slightly dodgy company, but their QC has improved a lot in the past couple of years. If you prefer a bassier sound and can stretch to around $15 or so, the QKZ HBB is excellent (QKZ are not related to KZ). Unfortunately it also comes with a pretty crappy cable, and once again the QKZ T1 is the solution.