Cibies and Hellas are good but expensive. The Volvo branded halogen replacements, they have Volvo and a big H4 moulded in the lens are pretty good too at less cost. The usual suspects or even a Volvo dealer should be able to supply. You will need new sockets for the H4 bulbs. Relays, correctly wired in will keep the volts up and H4's work at their best when fed the rated voltage. Many have converted without relays and not found much improvement over bulbs which aren't so fussy over voltage. Some H4 bulbs can be had that have a higher output rating for the same watt ratings but that comes at the expense of shorter life. An extra 10% a good compromise but you will get at least that improvement with the relays.