When looking to buy men's ski salopettes, note that quality is of the utmost importance. After all, you wouldn't want the trousers to become ripped or damaged during their first use, only for you to be left in need of purchasing more for your next ski trip. Look for salopettes that are made from a material with a waterproof membrane; this will keep you protected from the elements. Also ensure the product is suitably insulated; no salopettes are worth purchasing if they won't keep you warm.
Two good makes of mens ski salopettes are Bullet and Spyder, both of which never sacrifice on quality. These two brands produce salopettes that are suitably waterproofed and insulated and will last for years of use, without costing you the world. They also usually come with an integrated snow gaiter, which prevents snow from entering via the bottom of the trouser. Expect to pay in the region of £80 for one of these two brands, but special offers and clearances abound online; you can often pick them up for as little as £40. Lower quality, budget salopettes are available for as low as £10. It is best to avoid them though if you are looking to keep both dry and warm (sadly, with salopettes it is usually a case of you get what you pay for).
In terms of where you can purchase these items, you have a number of options. Firstly it is probably worth mentioning that you will want to avoid buying used skiing clothing in general. While the seller will be expected to provide an honest description of the product if it is listed on eBay, worn ski salopettes will ultimately fail to sufficiently protect you from the elements whilst on the slopes. Instead, head to websites such as Mountain Warehouse and Trekwear. Both websites sell salopettes for an affordable cost that ranges from £30 to £70 (at the time of writing, Mountain Warehouse has a pair of aforementioned Bullet salopettes for £39.99 that are on clearance, to give you an example of affordability).
Alternatively, head to Google Shopping and do a search for men's ski salopettes, where you'll be able to do a price comparison of websites that stock them, allowing you to find the least expensive at the time.