You use a little dab here & there that would all amount to a fat pea total. It will spread, especially the cream kind. I don't use more than that for my whole face.
I wouldn't mix it with a moisturizer; many moisturizers have ingredients designed to sit on the skin to hold in moisture & create a protective barrier of sorts. It could effect absorption. You want to make sure it's absorbed before applying a moisturizer on top.
Your doctor would likely prescribe it to you. Many, many, many women use retin-a for wrinkles & discoloration. Just ask about it. Can you go to a dermatologist specifically?
Most dermatologists will start you on .025% which is the lowest I am aware of. The highest available is .1%.
Are you in Canada or am I thinking of someone else? I get mine from an online pharmacy in India w/out a prescription

, and it's legit, but they don't ship to Canada because of some laws there. I always send money orders too, because I don't trust their site with my credit info (not secure). It's under $5 a tube + $15 shipping or so. There are other sites out there too that don't ask for a prescription, but you want to investigate them thoroughly to make sure they're legit. If you've found somewhere to order it online, then I'd suggest getting a .025% tretinoin cream (tretinoin is the generic name).
When I had health insurance, it only cost me $10 a tube, but it's like $75 a tube without insurance (in the US), and you have to have a prescription of course.
If you want to try an over-the-counter retinoid (ie. most commonly retinol) instead, then this page has a useful chart with products listed for each kind of retinoid:
http://www.skinacea.com/retinoids/types-of-retinoids.html
Note: it doesn't contain retinyl palmitate, which is weaker than retinol, but it's in many anti-aging products (mainly for show; it's not super effective because it's so weak & most products don't include a high enough % of it).
I don't know how well an OTC retinol will work for fading discoloration though, and many don't tell you their strength, so it's hard to pick an effective product. They also use varying names on the labels for it, which the chart explains.
Anyway, don't want to bombard you with too much info. Hope that's useful.