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Plenty on eBay and Amazon. The one I use is a KEF Concepts which works fine. Remember you will have no AF, no feedback of aperture and will need to stop the lens down manually. That all said vintage lenses are a bargain and you can get a very good lens for not much money. For example I paid £15 for a Carl Zeiss Jena 135mm f/3.5 lens in m42 fitment and it is pin sharp even wide open. Likewise Minolta MD 75-200 f/4.5 (£25), Minolta MD III 35-70 f/3.5 Macro (£20) and Rollei (Zeiss) 50mm f/1.8 HFT (essentially a Rollei QBM version of the C/Y 50mm 1.7). Minolta in the 1970s worked with Leica and both the MD lenses above are identical optically to their Leica SLR counterparts at a fraction of the price. Likewise Zeiss worked with both Rollei and Contax (though Rolleinar and Yashica lenses are not the same as the Zeiss ones).