Most garden centres only stock two or three common varieties of erysimum in the springtime. Typically, these will include cultivars such as 'Apricot Twist', 'Bowles' Mauve', 'Fragrant Sunshine' or 'Sunburst'. These are grown in huge quantities and retailed through garden centres, DIY stores etc.
However, you can find specialist nurseries that supply either a bigger range, and/or more unusual varieties through the RHS Plantfinder service.
If you are looking for a particular and unusual taxon, then do use the 'advanced' seach facility on your browser, so that you are searching only on the exact name.
A good way to buy some species and varieties is as seed, rather than as plants - just search on 'erysimum seeds' to find the companies and organisations.
Also, don't be afraid to buy directly from small producers via ebay. I've found good bargains and unusual varieties this way.
If you are unable to find the variety you are looking for, I may be able to help. I have to carry out a lot of propagating to keep the National Collection going and I often have some spare young plants to sell. If not, I am always happy to take cuttings and grow on young plants to order.