Have you looked at your local shops? Maidenhead Aquatics sells shrimps, though different branches sell different colours. Maybe give your local shops a ring to see what they stock?
If you want a specific type, you do need to buy on-line. I know that ProShrimp isn't selling shrimps at the moment because they don't trust couriers to get them to you overnight.
There are Ebay sellers, look at the seller's feedback if you consider buying from there. And bear in mind that some sellers make their shrimps sound better than they really are. The only other on-line shop I'm aware of is ProShrimp - when they start posting again.