Well, first you have to know if the Trustee will approve the purchase, and usually the Trustee will need information about the purchase in order to make a decision. Your attorney should talk with you about all of this.
To answer your questions though, the finance guys at most dealerships are going to have a good idea as to which if any of the lenders they deal with are going to even consider financing you while in a Ch 13. Most of our clients ended up going through the "Buy Here, Pay Here" dealerships because they were more likely to finance once the Trustee approved the purchase.
It's hard to say how difficult it will be to find a lender who will finance you, it's just going to take checking with multiple lenders until you find one that will. I would suggest that when you are looking around, make sure the salesman knows about you being in Ch 13 before they start running any numbers with their finance dept b/c that will save you time if some of them say no right off the bat.
Just a side note, very few of our clients who were approved to finance a vehicle while in CH 13 were unable to find a lender. It's possible, just may not be with the type of lender you would normally deal with.

