Can I buy 1 thing from Envato Elements without having to sign up for a monthly plan?

Can I buy 1 thing from Envato Elements without having to sign up for a monthly plan? If so, how?

if it’s for sale on the main marketpalces eg. themeforest, videohive etc.

no, you can’t. Envato Elements don’t sale product. Envato Elements provide a subscription base service where You will get unlimited download features only on Envato Elements.

Envato Market (as like themeforest, audiojungle) where you will find many more Items based on your needs. In Envato Market you will get plenty of Quality Items from different Authors. If you like, you can certainly purchase the Item license from Envato Market which will come with 6 months of Author support.