Hi,
Sorry they cannot be sealed as when they start tp leak it is invariably due to corrosion.
When mine went I was also quoted large sums to repair it....
Look on Ebay type in range rover aircon or even just air con - I bought a brand new one from an aircon firm advertising on ebay mail order for £150
Fitting is an easy DIY job - recharging can be done by any Decent garage.
Whole cost less than £250
Good luck
Wantaquad