I can't find metal bushings around here in Brazil.
Bushings in commercial jigs would be made from tool steel to give a long service life (months or longer in commercial use, decades in typical use by a home woodworker).
For a shop-made jig though anything that adds wear resistance can help, so a bushing can be any suitable metal pipe with the correct internal diameter1. Although copper is quite soft it's worth using if nothing else is available, ditto aluminium. Brass is better. Steel is best of all naturally, including stainless and mild steels.
If suitable pipe for a bushing can't be found and you want to reinforce then a surface plate of metal will also add useful strength.
How can I make a decent jig without these?
For light/occasional use and one-off jobs drilling jigs made from wood only work fine. Even softwoods can be used and give a useful lifespan. I made a pocket-screw jig from this plan from pine or possibly a type of fir, I can't tell which, and after numerous uses it hasn't worn much (but I am careful not to lean the drill when using it).
Harder wood is better of course. If you want to make a jig that lasts longer it is worth using a good strong hardwood, and since jigs of this kind only require a small piece even a really nice species that you wouldn't normally consider for a workshop jig can be used — very short offcuts that would be suitable are generally not of much use otherwise.
There are numerous dowelling jigs posted on YouTube2 if you haven't seen any already, here's a good one but there are others worth looking at, including some self-centring types.
1 If you're buying pipe online internal diameter is often shortened to ID in technical specs, and outside diameter to OD.
2 As well as guides on how to make dowel plate so you can quickly and easily form custom hardwood dowels. These two jigs make a very useful pairing for do-it-yourself dowel joinery.