It is difficult to find a travel agency which include information regarding accessibility. A few have tried yet there are so many variables depending on each individual's needs the task is certainly not as easy as one would imagine. If you happen to see a wheelchair sign posted on a travel brochure it's best to ask and confirm what they mean by accessibility. Do your homework before leaving do not leave it in someone else's hands.
Prior to embarking on our first trip I described my requirement for an accessible resort. Our travel agent contacted the tour operator who confirmed the resort we were considering was in fact accessible. I was fortunate to be at the travel agent when he called because his idea of accessibility was completely different from my own. Grab bars installed here and there just wasn't good enough for me I needed to know a lot more.
On our first and second trip I could get around using a cane for short distances. It was also possible for me to board the bus which would take us to our resort upon arrival As the disease progressed so did the difficulty boarding buses, when it became impossible we found finding wheelchair accessible vehicles very difficult and in many cases basically nonexistent.
To obtain the information I required one of the best methods I found was to communicate with others in an attempt to zero in on the ideal resort. Also important is to keep in mind these places are in a constant state of flux so it is a good idea to exchange information with someone who's been there recently.
With a simple search on Google you can locate thousands of travel sites, many of these will have places where people can post their reviews about the resort they visited.
To give you an idea take a look at Debbie's Dominican Republic Travel Page, it is specifically targeted for travel to the Dominican yet I have also read some information on resorts located in Mexico. You can literally sit for hours reading everyone's opinion about the resorts. I would copy and paste all the reviews into my word processor and then perform a search for some word identifying items of importance to me such as wheelchair, ramp, etc believe me it saved a lot of time.
Next on my list was a site named Webshots. There are many other sites that offer webspace for people to upload their photos. The greatest advantage with this is the opportunity to view the resort viewing pictures taken by tourists who have been there, not from the glossy brochure. I found pools which had ramps for wheelchair access and pictures of rooms with wheelchair accessible showers. Things you would be hard pressed to find on a travel brochure.
Bottom-line is do your research before leaving, the last thing you want to find upon arrival is that "oh yes we are accessible there's only one little step!" Or yes we have one room which is accessible it is being renovated right now, should be beautiful next week.
On a closing note CAA can probably provide you with travel insurance you require even if you suffer from a pre-existing condition like MS .