If you want to stick to purely Zeiss, the Orthometar 35/4.5 also works on the IIIa and is an outstanding lens (as is every Zeiss lens I've used so far). These are a little less common and usually expensive, but not that much more costly than a Biogon or Planar. The Orthometer is an uncoated prewar lens, but the lack of coatings seems not to impact its image quality - it's quite a piece of engineering too. Most Contax lenses are not rare, but a little uncommon, although there are usually some examples of the main ones for sale at any time on eBay or from the usual notable vintage camera dealers. Some genuinely rare ones do show up from time-to-time, such as the Old-Delft 35/3.5, even at prices that aren't too scary, but it takes patience to locate them.