Hi cat grass is usually oat grass you can buy small bags from ebay. I also buy wheat seed as wheat grass grows very quickly you can get first cut in about 7-10 days. This is also available on ebay. Although i get a 500g bag of seeds for a £1 from a bird feed supplier. Both these grasses are treat grass though. If you buy ordinary grass seed id recommend organic as some have fertilisers added and I wouldnt risk it being chemical fertilisers. The standard grass from wilkos that you weigh out yourself is fine though.