I am on my way to becoming a saddle/saddle pad expert with as much trial and error I have gone through to get the perfect fit for my HARD to fit mare.
I do not do cheap pads. Learned the hard way. Cheap will not get you anywhere but more money down the drain so you want to shop for a good quality pad in the first place. What we need to know is the style of barrel saddle you have (square skirt,round skirt,treeless,flex,etc) and then we can go from there. Also your horses conformation and saddle fit will help aid us in helping recommending a saddle pad :) Does your horse have a long back,short back? Built down hill?sway?,sensitive back? Mutton withered?.....For a good saddle pad I would get a classic equine memory foam pad. These do not come in a round skirt. They work really well but you do have to replace them within a yr if you do hard riding as the memory foam does compress and wears down(usually the shoulder area) and you don't get the most protection. However,I rode with my memory foam pad for two yrs and it was beat to crud but it still had life in it and still worked fairly well. Those run about $160 which is getting to the higher quality pads. Also,Beau said 5 star..these are great as well! If you can get a 100% wool pad it will be the best for them as it wicks away moisture and lets them cool down easier than fleece or synthetic matrial. You mentioned your pad slips side to side.If the saddle does fit then this has me thinking your horse is round and mutton withered and you may want to try a waffle weave neoprene bottom or neoprene in general will help. Twenty X pads are really nice. They are closed cell foam I believe and so they don't squish down like the memory foam. I really like these pads! If your horse is built down hill you may want something with shims like a ricotti,sharon camarillo,csi pad. Ricotti pads are great as the shim tapers down and it's not just this awkward lump upfront and then the pad...you need to make sure that you check the back of the saddle for too much added pressure if you do get one with shims only in front as this can cause too much pressure on the loins and make your horse sore(i know from experience). CSI pads are pretty versatile. They have a flex plate and you can add shims where you need them and the flex plate helps balance the weight of the rider and the pressure points on the saddle are more equalized. They are awesome pads! Those are around $350 new...you can SOMETIMES find them used but usually they are such great pads they are hard to find used. With these pads you don't need to constantly replace it as well. ....OH,and wither cut outs are the best so there is not as much pressure put on the withers from the pad being "strapped" tight against them.