The paste provided is usually perfectly acceptable. You can buy a small tube though, around £5. MX-4, ChillFactor III (the stuff provided with silver Arrows), Alpenfohn 'SnowCannon', Gelid GC-Extreme, NT-H1 (Noctua coolers), Prolimatech PK-1, ... It's all good, non conductive. MX-4 is easy to find, performant, cheap, and easy to apply (not too thick).