The 2015 Etape du Tour starts in Saint-Jean-de-Maurienne and ends in La Toussuire Les Sybelles. Ideally you would stay in one of the two of these locations.
La Toussuire is a ski resort 18km away from Saint-Jean-de-Maurienne so if you have a hotel down in the valley you can have a nice descent down the mountain post-Etape back to your hotel. Alternatively you could stay in La Toussuire and start your Etape morning with a 18km descent to wake yourself up. There is probably better accommodation available in the valley but if you are new to this sort of event and are likely to be on the point of collapse at the end of the Etape then you are probably best to book in La Toussuire, even if the accommodation is likely to be more basic (and there will definitely be fewer places to get a meal in the evening).
Unfortunately all the best accommodation will already be booked. What, already? Unfortunately because ASO, the organisers of the Etape (and the Tour de France), have designated travel partners, those companies get to know which route will be chosen for the Etape in advance and they then snaffle the best rooms and hotels in the local area.
My best advice is that as soon as you know your plans for the Etape - which days you need accommodation for and how many beds you need - the best way is to get in touch with the local tourist information office for where you want to stay. They should then be able to help you find the necessary accommodation. Work out in advance whether you want self catering or a hotel or a bed and breakfast and what your budget is. Then get in touch with the La Toussuire tourist office or the Saint-Jean-de-Maurienne tourist office. Bear in mind that if you are a large party (more than four) then you probably will struggle to get anything in these two villages.
For the 2015 Etape you may actually find more self catering apartments in La Toussuire as it is a ski resort with plenty of apartments. However they may largely be owned by French families who don't rent them out when they are not using them, so it's hard to know at this stage how much of a supply there will be.
If you can't find anything local, you could always try camping. This is obviously the budget option but it may also mean less sleep ahead of one of the biggest and toughest rides of your life, so depending on your fitness and previous experience camping may not be the best option for you.
If you can't find anything within the local area, you might want to check with the Modane tourist office. Modane is some 30km down the valley (towards Italy) but has been the starting point for Etapes in the past (most recently in 2011) and are quite well set up for accommodation. They may well be able to find you somethinf on the west side of Modane and then you would be faced with a short drive (or longish cycle) in the morning, as well as obviously having to get back in the evening.
You could also look for accommodation to the west of St Jean de Maurienne but the problem is that the roads around the town to the west are likely to be closed very early in the morning to cater for the Etape riders coming through, meaning that you may well have to be at the start line by around 5.30am if you want to park and avoid being blocked off. For that reason, I'd be slightly careful about booking anything in that direction unless you can ride to St J de M by bike in the morning and feel you will have enough energy to get back by bike in the evening when you're in meltdown!
Read our advice on whether you should choose driving or flying to the Etape du Tour and choosing an Etape travel tour operator.