I bought a Behringer SRC2496 to use as an outboard ADC when recording from cassettes, VHS HiFi, and other analog sources. It supports up to 24/96, has an input level control, and an LED level indicator. For a capture card I have an Audigy 2 ZS which does bit-perfect. I made my own Toslink receiver bracket for it, but I've since picked up the deluxe drive bay module that provides more convenient I/O for that card.

If you're in the EU, elv has some inexpensive but high resolution ADC and DAC units. Unfortunately, shipping them anywhere else is exorbitant.

There's also generic ADC little black boxes on eBay and Amazon; they may not be terrible. I looked inside one, and found name-brand chips.