Search "486 computer" on eBay and find one you can either use as-is or use parts from it - plus the case. The AT case is actually the hardest part to find these days.
You can source the other parts on eBay as well. Locally, recycle centers sometimes have what you're looking for, depending on where you're located.
I assume you mean MB, not KB on your RAM spec. 16-32MB is appropriate for a 486, less useful the further into the Pentium era you get, running Win9x etc.
Why the specific limitation on hard drive size? You can find those too, but be careful on shipping. Compact Flash cards of varying sizes (industrial cards <1GB and consumer cards >1GB) are popular because they have no moving parts and are pin-compatible with IDE. If you're experimenting with file compression though, you may want the limited speed of a spinning disk to keep you accountable in your code.
For SB16 cards I'd recommend looking for the jumpered type such as CT1740, CT1750, etc., rather than the Plug-N-Play types. PnP cards work fine and you may prefer them, but they are less fun. If you get PnP, use the UNISOUND utility to initialize them for convenience. Check this page for the model numbers: Sound Blaster: From best to worst
Standard ports on a 486 may or may not need to be added with expansion cards (usually Multi I/O cards with HDD/FDD, serial, and parallel). Some motherboards will have them built in, but will need breakout cables.
There is no such thing as "EVGA". What you want is a nice 2D SVGA card. S3 and Cirrus Logic won't do you wrong.
For 486 motherboards you have three options for your bus: ISA only, VLB, and PCI. ISA only is the worst for a 486 class system. VLB and PCI are what you want, but PCI is easier to work with. You will have to make sure your video card matches your bus.
For CD, any generic IDE CD-ROM drive will work.
You will find an Ethernet card and the MTCP software suite invaluable for transferring files once you have the OS loaded. The 3C509 series of cards are plentiful, inexpensive, performant, and reliable.
World's foremost 486 enjoyer.