Generally, when looking for harder to find parts such as this signal light my first thought would be to check online retailers like Rockauto, etc. However, this turned out to be a no go as this car has very few parts available probably due to its popularity like you stated. After this I usually hit up eBay just to see what can be found. Interestingly though, it seems that plenty of previous generation turn signal excel parts were available but not the one you need as well as plenty of brake light housings. Again though, no success.
After this, I'll work to track down an OEM part number. A lot of online "dealerships" - of sorts, at least for parts - have diagrams breaking down sections of the cars and providing part numbers for them. Specifically here I found that this website - Hyundai Parts Deal - had the parts diagrams I was looking for - Body Side Lamp - '93 Excel. In this case the part number is 92306-24350 for the passenger side and 92306-24350 for the driver's side - I wasn't 100% sure which side you meant by right so I provided both. Now, I'd always recommend you take these numbers and punch them back into google and confirm this is correct using other sites and images. If you want to be doubly sure, call a local hyundai - insert manufacturer - dealership and provide the part number and ask for a description.
Once you've confirmed your part number feel free to call a bunch of dealerships and check out online retailers - even places like eBay and Amazon sell OEM parts sometimes. Once you've found a good price at a reliable source, well you know that part.
Anyways, it seems the site I listed earlier is selling the part for around $31 before shipping, fees, etc. I would recommend you check out their return policies or warranty info if possible beforehand just in case.
All in all, best of luck to you. I hope this helps you get your car back up to a fully lit state.