fitting extra bus seats

Anything I should look out for? Unfortunately although our camper is a minicoach conversion the previous owners seem to no longer have the remaining original seats (after saying they did...). We rather desperately need at least another (but will fit 2 or 3). Can I just look on adverts or donedeal or do I need to bear in mind measurements or fixings? Is there anywhere that could do this for us given that we are short on time? (Bed will need to come out and be shortened in order for seats to go in). We are in Dublin and Wicklow.