Ugh, Denise Austin also happens to be a joke within the fitness instruction community! A bit of a ditz. (I've been a fitness instructor for 11 years.)
If you go to any type of gym, you could also do a stationary bike or elliptical machine for cardio.
You can get a decent treadmill used on Craig's list, although the one we bought we thought was decent - it's a bit rough for jogging, but fine for walking. I think we paid $150, but there were seriously cheap ones listed for less than that.
Definitely do videos! (Like on-demand or borrow from friends of the library.)
Do not limit yourself to those specifically geared towards prenatal exercise! 
Really, there is no need at all. Don't even limit yourself to low-impact. Any aerobics video or class will do - it's really easy to modify & do a low-impact option. I personally always make a point of demonstrating the low-impact option for any high-impact move I teach. But even if they don't show you a demo you can figure it out - i.e. instead of jumping-jack, just side-tap. Instead of shuffle, just step-touch 2X to the same side, instead of squat/jump up, just squat & stand tall (or 'releve' - go up on your tippy toes, but keep those abs locked to support the lower back!)
Try out all the options on the on-demand videos, just modify as needed.
You can even do step-aerobics videos without the step! Just pretend you're stepping UP onto the bench, & doing a knee lift, but you don't actually step UP. It's no big deal. As I tell my classes, in order to get a cardio workout, all you have to do is MOVE! Mastering the choreography doesn't really matter at all** - just keep moving.
Otherwise:
-Obviously no laying flat on your tummy
-You really shouldn't lay flat on your back either (if you do have an aerobics step, you can put 2 risers under 1 end to make an 'incline bench' for youself.)
-Careful stretching - don't overstretch, take it easy
-watch out for balance in case you feel yours going, avoid things like stability ball work (this never happened to me personally though.)
-careful about overheating (although less of a risk if you're exercising indoors & now that the weather is cooling down.)
That's really about it! Do any old workout you like!
& for resistance training, again, if you don't join any type of gym, consider getting dumbbells or tubing. Since you've been sedentary a while, you want to go lighter, higher reps, & ease into it- but it's still low-impact & great exercise! & upper body strength really comes in handy carting a baby around!

Feel free to PM me if I can help anymore (I'm also a certified personal trainer.)
**Not entirely true - there's some research that repeating patterns as in an aerobics class can help exercise the brain & stave off alzhemiers! But, as far as your cardiovascular health is concerned, mastering every bit of the choreography is irrelevant.