We stayed at the Belledone campsite, near Allemond, at the bottom of the Croix de Fer climb - about 4 miles down the road from Bourg D’Oisans. We actually stayed in a chalet, not a tent. You don’t need a guide or even a GPS file. The local tourist info has a paper guide to the top 30 climbs in the area with maps, which includes all of the iconic ones and a few obscure ones too. There are so few roads that you just ride along the valley, pick your climb and ride up it. Alpe dHuez, Glandon/Croix de Fer, Col D’Ornon, Les 2 Alpes, Lauteret/Galibier all lead off the valley.
PS - my profile picture is taken near the top of the Galibier. This was in mid-June and there was still plenty of snow that high up, though none on the road itself.The pass doesn’t open until early May each year, due to snow blockage.