Coverage can vary widely among travel insurance providers. It's best to carefully read their terms, exclusions, and definitions to understand what is covered and what is not for each company, and often they offer several types or levels of coverage.
That said, pre-existing conditions can sometimes also be chronic conditions, and several reputable companies offer a "waiver" of that chronic condition under cerain circumstances with slightly higher cost policies. Again, exact terms vary, but usually you must be healthy enough to travel for your dates at the time of the trip booking, not booking against a doctor's advise, and must purchase the insurance within a short time of making your initial deposit for the waiver to qualify (This might be at time of deposit, or within 7 or 14 days of your deposit or that condition will not be covered). A big factor is that the condition is stable and you have not changed medication, treatments, seen a doctor or should have seen a doctor for any complaints with that condition for a certain period of time (usually 60 to 90 days on policies I have seen) before the trip is booked and insurance is purchased.
If the condition is stable with no changes in treatment for the look-back period before you book your trip and purchase the insurance, you are generally covered for anything that comes up AFTER the policy effective date, which is usually the purchase date or the day after purchase. If you do NOT have a stable condition when you book and deposit your trip it may be hard to find coverage.
There are still some cases where coverage could be denied. It really depends on the type of condition itself, and on the insurance underwriters for your policy and the medical information they get from doctors and hospitals when a claim is made.
Travelex Insurance is a long time well regarded trip insurance company, there are many others.
