seems no one wants to answer. maybe everyone is hoping it'll stay 75 forever around here
some guidelines I follow (good for nj/ny/ct area - people in san diego would think I'm crazy):
<65 - ls jersey (any wicking type is good), wicking baselayer (could be a regular ss jersey). shorts should be okay. short finger gloves. maybe a cycling cap.
<55 - possible knicker or knee warmers, shorts, ls x 2 jerseys. long finger but not "thick" gloves, i.e. summer mtb gloves would work. cap is good or a thin head cover that leaves ears and neck uncovered.
<45 - knickers or tights (if dry), booties (shoe covers, I like the ones I have, forget the brand but it's just a long strip of velcro in the back, no zipper), thin but insulated long gloves, wind vest, 2 x ls jerseys (or ss + windproof jacket), head cover (I use some polypro tubes that fit over your head). taping up your forward helmet vents helps.
<35 - tights, booties, windproof jacket, ls baselayer, insulated gloves, some ear/head/neck covering. deluxe would be insulated/winter shoes or heated insoles, heat pads. tape up your helmet vents or buy a cheapo helmet with less vents.
<25 - I don't ride outside unless I have to. add a pair of shorts or even a second pair of tights (but total one pad to sit on), extra baselayer, warmest neck/head covering.
if it is cold, you should feel cold when you start. you'll feel a lot better once you warm up. if you are riding relatively easy, add 5 degrees to each rating. if riding hard (race), subtract 5 deg. if you're riding off road, subtract 10 degrees. if in san diego, add 15 deg
(actually I go to SD in Feb to get early season miles as the weather is usually great there so nothing against SD)
since "warm" is so subjective, I jot down in my training diary what I wore, the temp, and how it worked out (too hot, too cold, not enough wind resistance, not enough breathing, etc). I look back to see how I dressed on various days and use previous rides as a guide. in the fall I use more gear for a given temp than in the spring.
rain makes everything pretty heavy and cold - I'll under dress a bit in order to avoid lugging around 5 lbs tights or a 15 lbs jacket. I use some thicker rub (vaseline for example) as a substitute for having tights and wear knickers or shorts instead.
If cooler and using appropriate covering gear, I'll use heat rub (atomic balm, not the weak store stuff) on my achilles and knees and put my socks/booties and tights/kneewarmers respectively. I've been known to put (black) duct tape on my gear to create an outer windproof layer on my gear.
where to buy this stuff? home depot for duct tape
but seriously... baselayers you can get from any place that sells ski stuff. or a bike shop. Ls and ss (long and short sleeve) jerseys are best bought at a bike shop or some swap meet or, if you have friends who ride, get hand me downs - really serious riders have way more clothes than they know what to do with. if patronizing a local shop, see if they have club gear, maybe from last year, that they need to get rid of. or join a club - many of them give you a jersey in exchange for joining. knee warmers are a shop thing, as are cycling tights and a windproof jacket (get the "winter" kind, not just the windproof shell kind). cycling gear is cut long in the sleeves, the back, and the fronts of legs. they fit great when you ride and not great if you're not riding. a wind vest is invaluable but can be replaced by putting newspaper or plastic bags under your outer layer of clothing.
in order of importance:
1. baselayer top (one that is black, ls, and thick will work on its own and also as a base to another jersey - I'm assuming you have a ss jersey)
2. booties - shoe covers - insulated, not the aero covers time trialists wear
3. effective long finger gloves (specialized makes a thin insulated set usable down to sub 30 degrees)
4. tights (get thinner ones if you will skip the outdoors below 40)
5. wind vest
6. winter jacket
*always* carry an extra layer with you, just in case, and bring energy gel/bar/water/whatever. if you run out of fuel, you'll get really cold really fast.
also I'd recommend head and taillights (blinking LEDs work fine) as it gets dark pretty quickly.
if you need small sized gear, I have tights (thin and thick), wind vests, wind jacket or two, jerseys (ss and thin/thick ls), and I'm sure a few other things I can throw your way. no jackets, booties, knee warmers sorry.
