Generally with 3000psi cylinders they don't often get tested as they can be bought new for about £30 and testing could be £25, but can be as cheap as £15 so may be worth your while.
a local dive shop can probably arrange testing as long as their mark up isn't too much (or you can find an authorised test centre nearby)
I had 2 different experiences with a local dive centre. First visit was with a fibre cylinder and I got a good price, just a bit of a wait due to the winter season but I wasn't in a hurry
On my next visit I asked about testing my 3000psi cylinders and basically got the wrong guy who didn't like paintballers - I think he quoted about £40!
A thing to note though. Depending on age and the type of cylinder if they are aluminium which most 3000psis are (despite generally being called 'steelies') then they have a life of 10 years before testing
If they look in decent condition, are aluminium and the local site accepts the extended life then you may not relay need to get them tested