PhD Student here. I came straight of undergrad, but I think I can help you.
First of all, the title of your previous degrees has very little to do with what you can or can’t do. Especially at the grad-school level. Urban Planning is fine, especially if you have an interest in getting into the the field of Urban ecology or conservation architecture. More pressing is your ability to show a passion and interest in the field for witch you are applying. For example, My undergraduate degree is in Psychology and my honors is in health science, but my PhD is in computational neuroscience in the invertebrate visual system. One of the other members in my lab did her undergraduate degree in physics and then masters in particle physics before deciding she wanted to do a PhD in Neuroscience. All the previous degree has to do is prove you can work hard and be motivated by intellectual curiosity, you can let your passion do the rest.
As for how to find a program, a lot of the advice I see given is things like e-mailing or phoning professors whose labs you are interested in. Yes, networking is important, but it i’s not the 80’s anymore. You don’t need to do cold calls or send out hundreds of e-mails, you can let the projects come to you.
First, go sign up for Twitter. It can be pretty daunting, but I’ve had it for about a year and see several post-grad positions offered weekly. Once you signup spend a bit of time learning how the site works & so on, then just start following people in the field. For example, If you are interested in marine ecology, search the #MarineEcology tag and see who posts in it, Follow them, then look at who they follow and so on. If you are interested in ornithology, search that. If you want somewhere to start, follow me @ benjamin_lancer and look at the people I follow. Almost all are Neuroscientists, Ecologists, Ethologists, or people working with animals in some other capacity. Once you start following people, just sit back and enjoy the animal facts/pictures/whatever that rolls into your feed. The thing about Scientists is, when someone has a position open they will probably post it on twitter (Usually this is a position for an already funded project with set research aims), so if you start following scientists who work in areas you are interested in you’ll eventually come up on those posts (As I said, I see a few every week and wish i could do a new PhD for every single one). Then it’s just a matter of reading their post, googling the professor to get their e-mail, and contacting them directly. Say you head about the project on Twitter, are interested in it for XYZ reasons, and and fingers crossed from there.
I wish I had known that when I started my PhD, because then I probably would have found a project far closer to my personal interests. I pretty much started the first project I saw because I was terrified of not being able to find anything/that no one would accept me, but now that I know where/how to look, there is an abundance of projects to apply for. Networking is still key, but it’s not 1980 anymore. There is no reason to send out cold calls or unsolicited e-mails when you can let projects come to you, and then respond to the ones that interest you.
Another good website is www.findaphd.com. Sign up, input your interests and where you would like to do a PhD (From nations to individual universities), and it will return a list of positions professors have put adds for on the website. My only warning is I find a lot of professors forget about this site, so there are lots of programs not advertised there, and a lot of positions are filled but never updated on the there.
I honestly think scientific circles in social media are the best way to find a place in modern academia, so my suggestion would be to just start hanging out in those circles, enjoy all the cool animal/ecology discussions that arise, and see what comes your way.