(1) mymedsbuy is one, alldaychemist is another although recently there was a post saying ADC no longer takes credit cards.
(2) I sure can't tell any difference between the $100/pill USA Pzizer and the $1/pill Indian ones. Sildenafil citrate is the same no matter where it is produced. I'm 62 and don't get as hard as I once did, but 50+ mg of Viagra and I get boners like I did as a teenager, so hard my cock twitches with my pulse. This is NOT the case for everyone, but damn it works for me!
(3) No prescription required. Cost is LITERALLY under $1/dose. Buy Viagra in quantities of 20-40 100 mg pills and split them. You probably only need 25 or 50 mg, so that's 80-160 dose. Delivery is extra, but not outrageous and it still comes in at $1-2/dose. Delivery is slow, 3 weeks or so. One order was "seized" by US customs and I got a letter to that effect, but ADC reshipped no questions asked and the 2nd order had no issues.
You might want a P.O. Box if there are issues for you with DW asking about "The package that came from Mumbai".
There are (for me) mild side effects; a little nasal congestion, cialis can cause a little muscle pain in the lower back. These are mild issues and easily addressed with Advil and/or a decongestant.
Large dose of viagra are often reported to make it a bit harder to reach orgasm. I'm finding this to be true, although that wasn't true when I was in my mid-50s. On all the side effects, its YMMV and you just have to experiment.