I feel that Pilates is a lot like Yoga as far as getting certified is concerned. They are both very deep movement programs and require a solid and comprehensive study program. I can’t see how weekend cert programs and online programs could prepare an instructor properly. I would find a program nearby that is very indepth with a well trained instructor who has a verifiable background working with a well known Pilates Master. If you don’t have access to such a program nearby, I would consider going to a specialized school for an extended vacation. It may be pricey, but you owe it to yourself and your future students.
Do you still teach for the Y? The Y does have a Pilates training class, but it can be hard to find depending on your area. I do some teaching for the Y in my area, but I am not certain if your organization follows the same structure, but if you teach at the Y the trainings would be found online. I know I looked last year and only saw a handful, and none very close to me.
If you are not going through the Y you are probably well served going to a well known group such as PMA, or PhysicalMind institute…. there are a few that are really well known nationally….
Here is a link to IDEAs tool to compare different Pilates certification programs: http://www.ideafit.com/certifications/pilates
Now if you just want to add some pilates techniques into what you do already you could opt to do a few workshops and find the concepts and movements that would work in your class formats. I would say if this is what you want you should consider taking a workshop with the best presenter you can get to and can afford, and that will give you more than spending the same amount for an online certificate that may not get you the industry respect or the hands on experience.
The training is expensive and takes a while, but what I have seen is that the people who work specifically in Pilates take their training quite seriously, and to work in a top studio you need really top notch training. And while the structure of how one trains in Pilates is quite different than it is in yoga (you say you teach yoga so will be familiar with that) the trainings do mirror the complexity of each discipline. Baseline in yoga is the 200 or 500 hour training (to get on the Yoga Alliance, which is the industry standard) …. and I think if you want to do the upper level equipment training in Pilates you are likely to look at training that is similar in commitment.