All Rhino parts drop from the same boss on a Venus node; issue is that you don't know what you get so you might have to run that dude 10 times or more if you're unlucky. Boss node(s) should have a special icon.
I'm not sure I'm quite ready to join a clan yet, but does anyone have a particular YouTube or Twitch Streamer I can watch that has a good introduction/training series on Warframe? I'm new and wouldn't mind watching some vidoes that give some good ins-&-outs/do's-&-don'ts.
Mogamu on youtube, then probably check his recommended channels on his page and the related channels section.
Some other people would be Tactical Potato, Brozime, AGayGuyPlays and DKDiamantes.
On twitch the list is probably similar, just check the most watched I guess. I'd say Warframe doesn't lend itself well to being streamed so there aren't that many large streamers covering it.
From what I understand you don't deal enough DPS. Post a picture of your layout for current standard primary you use. Makes it easier to spot stuff.
For primaries: Mods called Serration and Multishot are your next big goal. You need those and you need to start leveling those up, Serration has probably priority over multishot, till you get +100% damage or so. Somewhere on the internet a nice person did the math on how to maximize damage for resource strapped players.
Against corpus mod for Magnetic (electricity + cold) and Toxin damage, or just plain Toxin.
Wiki and the in-game codex show you how enemies react to certain damage types.
Most weapons fall into certain archetypes - crit weapon, status, crit/status and just pure damage.