You're gonna want to do a huge amount of research on whatever you buy & the sellers and you'll also probably want to talk to your optometrist about it - generally, 'costume' lenses arent made to as high a standard as medical contacts which can potentially lead to some issues (And your optometrist will no doubt pretty much tell you not to do it, but their job is to inform you of the dangers and let you decide).
Firstly, just on a basic level, you'll want to do some reading on safe brands & types of 'cosmetic' contact lenses.
here's a short article detailing basic contact lens details,
here is an article listing brands of contact lenses (supposedly the top 3 are FDA approved, but I would double check that), and
here is an article from the FDA. I'd also be a bit remiss not to link some of the easily found horror stories - these were what turned me off, honestly - but dont take it as me trying to encourage you not to, just trying to make sure you're aware of the fact that these injuries can happen very quickly and be permanent.
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Now! Onto more relevant to chiss stuff! You're going to need sclera lenses if you want the full red eye. Now, I dont actually have much experience with these, but all my reading & memories of what I was taught say that they can be even more dangerous, very uncomfortable & dry the eyes out significantly, and you should only wear them for 4-6 hours maximum and not multiple days in a row. A proper set can also cost you upwards of $300.. Think you can tell why there arent very many chiss around now, it's quite expensive! There are some great Thrawns out there who you could ask what they use - you basically just need to google 'thrawn cosplay'.
Good luck!
