In Russia, Kazakhstan and Belarus
Russia
You can buy them through a brokerage account or an individual investment account (ИИС, IIS), and "people's" OFZ-n bonds (ОФЗ-н) at some banks. Coupons are subject to personal income tax (НДФЛ) at 13% (15% on the part above 2.4 million ₽). OFZ are not covered by deposit insurance, but are considered the most reliable rouble securities. Their market price may fall before maturity.
Kazakhstan
The equivalent is government securities of the Ministry of Finance of Kazakhstan; individuals buy them through brokers on the KASE or AIX exchanges.
Belarus
The equivalent is government bonds of the Ministry of Finance of the Republic of Belarus, which are sold to the public through banks, including in foreign currency.
Sources: Ministry of Finance of Russia: public debt («Минфин России»)