It's a crap shoot, but a lot of times you just have to ask. This isn't just confined to dropshippers -- wholesalers and even affiliate programs can make their resellers' lives way easier and time spent more productive by offering a CSV or similar format file of inventory. Would that more did.
On the other end of the business, DataFeedFile and GoldenCAN are both highly useful for bypassing the code and creating multi-vendor product feeds. I'd love to see some kind of wholesale product list that could do the same thing, drawing data from multiple suppliers and creating a product feed automagically.
In the meantime, if you ask a supplier for a CSV, you might find that they have it available even if they don't advertise it as a perk. I mean to say, they obviously have the data, or they wouldn't have a website. Doing some product testing on a small scale and getting in a couple of orders, and/or getting a specific individual with whom you deal at the supplier, is often the key to getting unadvertised assistance in your efforts. A given supplier might also have it available for established customers only (since as I'm sure you can imagine a lot of dropshipper site visitors -- over 90% -- never buy, and a lot of those -- probably another 90% -- never come back.)
"Failure is not when you fall down. Failure is when you don't get back up."