I have a bowl of assorted goodies for clients to choose from. I generally 'stock' Kimono MicroThin Large (they run a bit small, so large fits normal guys - I keep "regular" on hand for smaller ones), Kimono MAXX (in between a regular condom and a Magnum), Crown Skinless Skin, Durex Extra Sensitive (best for intact guys, as they're made in England where virtually everyone has a foreskin), Trojan Ecstasy Magnum XL.
I would use someone else's condom only if it was still in the sealed package. This has only come up when the client wants to use a latex free condom, as I don't tend to keep more than on or two on hand.
I buy from Condomania and have been for 3 years. They send tons of coupons, I think I pay on average .40-.60/condom