Hello Helen. I'm in France. Not hinthered by much expérience with agrumes, i'll " advice " you.
I'd go
online and check Portugese websites of tree nurseries.
Maybe just get rootstock! And otherwise get the whole tree.
I grow rootstock myself. Of apples, pears and cherries that is... Not a big deal. Guess the same could be done with agrumes. Speak to orchard folk. Just ask what you could do.
Sometimes in old discarded orchards the trees fall over. That could be a source of rootstock. I doubt they care much.
But maybe no rootstock is needed. It seems following YouTube tutorials i saw they just grow easily from cuttings.
I checked that out because i'm in the south of France now where in the middle of an eighties housingblock a Giants Pomélo stood full of fruit. The owner let us climb it with ladders and harvest a bit. Forty kilogrammes of fruit. There was loads more, but no time.
I took cuttings like this in bottles. Hoping i won't have the problèm of rootstock i just put fourty like this and have quite some seed.