There are plenty of places to look, e.g. Pony club websites or word of mouth from other PC families, horsequest, horsemart, H&H magazine, etc. The main problem is being able to correctly judge if a pony is suitable or not. To be blunt many people will lie and will try to sell you a pony that is unsuitable for you and 'unsuitable' means can't or won't even be ridden.
Do you currently take lessons? Ideally your current instructor, who knows your abilities, should come with you on viewing and assess the pony. This will cost extra but it is money well spent. You still need to arrange a vetting, but you only arrange the vetting for the pony that you have established is likely to be suitable and once you have agreed on a price with the seller. While it is true that some ponies may well fail the vet and then you are back to square one, at least the vetting saves you future trouble. You should also take out vet insurance (as well as liability insurance) as any pony may incur costly vet fees at any time.
The best way to get a pony is by word of mouth. If you find a family pony that is well loved but outgrown you have the best chance of getting an honest history and a better chance of finding a good match. However, reliable PC/hacking mother/daughter share ponies are rare and will command a good price even in this market.
Another option is to go to a dealer. Some dealers have a, deserved, rubbish reputation, but a dealer with a good reputation would help match you to the right pony. If you buy from a dealer you have a much better comeback in case the pony turns out to be unsuitable (although in the case of a dodgy dealer it may be a nightmare returning the animal and getting your money back so it's always worth going to someone with a good reputation to uphold).
It's always possible to find the right pony through private ads but you will need to go see a lot of them and go in with your eyes open. Ask to see the pony caught, handled, tacked up. Ask to see it trotted up to exclude the obviously lame (yes there will be some like that!). Always insist on seeing the owner ride the horse first, if they refuse walk away (yes some owner are entirely negligent and will let you ride their totally insane animal in the hopes that it will behave itself for half and hour and you will buy it). Always ride before your DD to assess the pony. If at all possible ride in a safe, enclosed area first and if all goes well (walk, trot, canter on both reins, put a couple of jumps), ask to take the pony on a small hack. Try to see what the pony is like in traffic, even if you have to ask the owner to drive her car and tractor around the yard (make sure the pony is happy to be overtaken by the vehicle, not just stand still while it passed infront of him). Check for brakes in a field (ideally with a snaffle mouthed pony). Try to see what happens when you apply a bit of stress, e.g. walk the pony away from the yard, walk it back and half way back home turn around again and see what happens.
Anyway, I am rambling now but I hope this helps!