Machine translation:
Hello!
Does anyone know in Moscow or Moscow Region a person or service center (from where you can call home) who could replace the thermal paste in a mid-2010 iMac and explain to me what to buy to add RAM (there are two free slots, but I don’t know what to buy).
As you found Apple's English-language forums I doubt there will be very many other users who can help with recommending a Moscow service center. However, overheating problems in iMacs are far more likely to be other hardware issues, workstation layout, or software interferences than a thermal paste issue.
RAM: these are the correct specs for RAM for any 2010 iMac:
https://eshop.macsales.com/item/OWC/1333DDR3S08S/
That is the vendor I use. Although US-based, they have business partners in many countries.
https://www.owcdigital.com/reseller/resellers
However I do not see any in Russia listed. You can also see if crucial.com shows RAM availability in Russia. Avoid bargain RAM.
Although Apple does not have Russian-language forums like these, they do have other European language forums. One may be an option for future questions if posting here in English is not feasible:
Spanish: https://communities.apple.com/fr/welcome
Portuguese: https://communities.apple.com/pt/welcome
German: https://communities.apple.com/de/welcome
French: https://communities.apple.com/fr/welcome