compte individuel online?

I don't remember by heart if it's on there, but you could try Tax-on-web if you have access to it.

If not, contact the sociaal secretariaat your employer was affiliated with (for example, Partena, Attentia, SD Worx,...). The name should be on your pay slips. Normally they are not allowed to provide documents directly to employees, but they often make an exception if the employer no longer exists. Note that you will probably be asked to prove your identity somehow, especially now that GDPR rules are about to take effect.