You should definitely grow your own because you never know what you'll get from a dealer.
If they're getting it second hand, they don't even know what they have most of the time.

For true medicinal use, you need to do a little research to see what strains are best for muscle spasms & pain. If you end up with a trippy head high, it might be fun (or not), but it won't help you feel any better.

We're only shipping seeds within the UK borders right now, but if you click on those little links that show up under your original post, you'll find a couple reputable places that will ship to the US. Most of the time, the seeds do slip past customs - but there's always the risk they won't. Look for a supplier who will reship if they don't.

Here are a few to check out -
CBD Crew - Very new seed bank that specializes in high-CBD to THC ratio seeds. This means that the "high" factor is tempered so that the physical powers are more intense. Supposed to result in a more medicinal weed.

Medical Seeds - This breeder is a little pricey, but they have some really great medicinal strains. Hammerhead looks awesome to me - but I haven't personally tried it.

Some say White Rhino is a great medicinal for physical pain, but there's some controversy over which breeder to go with.

Here are a few things to keep in mind while you're looking
1. A lot of strains are adverised as medicinal when they really aren't - it's because everyone wants to market them that way right now.
2. Medicinal strains can target physical or mental symptoms. For physical, you generally want to stick with Indica or Indica dominant strains - although a little mental lift from a Sativa isn't always a bad thing if you pick one that lifts the mood without getting you high as a kite or making you anxious.
3. A lot of medicinal patients go with edibles. Cannabutter sounds pretty great - and there's an awesome post about it on this site. Vaping is a good choice too. Lot's of options to pumping smoke through your body these days.