I would recommend looking at Solution saddles. I have one for my very wide, round cob and have never had problems with it slipping. They are quite expensive compared to other treeless saddles but you really get what you pay for.
Avoid the very cheap ones, make sure you have the correct pad for it and see if you can try before you buy - some people hate them.
I have the Barefoot London - very comfortable and you can buy it with the very wide pommel - for table top backs, though I have the standard one which is perfect for my gypsy cob and clydesdale.
I have both treed and the treeless which I use on our two. I keep the treeless now in reserve as they are both still growing so I can still ride whilst finding a new saddle. The treeless is also very comfortable and relaxing to hack out in.
Definately avoid the cheap one's on ebay!! Only one's of a 'budget' price I'd recommend are the Libra range you can buy on Equine Whispers site...BUT I just got myself a bargain on ebay of a brand new shop soiled Libra Synthetic for only £77!!! I bought it because I had to sell my treeless to buy a saddle I could jump in but I missed hacking out in the comfort of treeless so kept my eye open for a bargain!!!
You have to ensure whatever saddle you buy you have a good padded saddle pad, stirrup leathers that adapt to treeless, dressage girth and ideally safety stirrups...so often buying the saddle is only the start of the expense!!
I had a Barefoot Cheyenne which I sold because it was impossible to jump anything decent in and also it was too long in the back for my short backed cob, but before that I had for 4 years Randol's Treeless from Germany and it was fabulous..still kick myself for selling that to buy the Barefoot but you live and learn!!
I can give you the seller on ebay I got the Libra from if you want, pm me and I'll send it to you. She was extreamly helpful and I'm sure will happily give you some information before you buy.
If you are looking for a cheapy one for pootling around I can recommend a second hand Barefoot or a Libra from eBay.
Other brands I can recommend are Solution, Heather Moffett's Fhoenix, Freeform, Freemax, Trekker.
All easily available from eBay if you know which suits you.
Of all listed only the Solution and Fhoenix don't require a specific pad with them. All the others should be used with their corresponding pad or an Equitex, Thinline or Christ Lammfelle pad.