It's illegal to keep or sell non-native crayfish in England. The ONLY exception is the Cherax quadricarinatus aka Red-claw crayfish aka Blue lobster. There are other species sold as blue yabbies, but you would be responsible for knowing the difference.
The following excerpt is taken from the source site below.
"The legislation was introduced seven years ago through the Prohibition of Keeping of Live Fish (Crayfish) Order 1996. However, many people are still unaware of the legal issues in this area. In a nutshell, if it's a crayfish, there's a 99.9% chance that you aren't allowed to keep it.
With the exception of one species, the Red-claw crayfish (often sold as the Blue lobster), Cherax quadricarinatus, they are all illegal! Even tropical crayfishes are banned, so do not be tempted to buy one should you see one on sale. This is why PFK no longer covers them.
Very importantly, there are several crayfish sold in the trade as Blue lobsters, not all of which are Cherax quadricarinatus. Not all blue lobsters are legal! Procambarus alleni, an illegal species from North America, is often sold under this name by shops who are unaware of the legislation.
The measures serve to protect our native White clawed crayfish, Austropotamobius pallipes, which has suffered as a result of the introduction of non-natives like the Signal crayfish.
The native crayfish has been suffering from a disease called crayfish plague, which was probably introduced by exotic crayfish that escaped from crayfish farms where they are bred for the table. (Crayfish are very tasty - Pret a Manger do a superb crayfish and rocket sandwich if you want to try it).
Strangely, there's a funny loophole here in that it's currently legal to import banned crayfish from the EU, even though they can't be legally kept upon arrival. The Fish Health Inspectorate is attempting to stop this trade and can remove crayfish from consignments and could visit premises to seize illegally kept specimens.
If you already own a crayfish and are unsure whether it is a Red-claw, there is an identification guide on the eFishBusiness website:
Before buying (or selling) a crayfish always check the above guide to ensure that the species is legal to keep in the UK! Many of those seen in shops are not."
To answer your question, if the local petfish shops don't carry them, ask if they would order for you.
Cost will depend on the availability and cost to the owner. Many may not be willing to order due to potential of getting the wrong species.
If that's the case, try an internet store.
At a potential 20 cm in length, it would need a larger tank than 27 litres.