The best advice I can give you is to collect your own seed from neighbors or friends at the end of the season when the seedheads are fully ripe and brown. I have tried ordering seed from EBAY, but none ever germinated, meaning that either the seed was old or was collected before it had fully ripened.I would never order from EBAY again.
Remember, if you are planting clematis seed from Large Flowered Hybrids you will not get the same flower from it's seedling. The flowers from these hybrids CAN be VERY different than the parent/s. This is how new hybrids are 'born'.
Raising Large Flowered Hybrids can take an average of 6-24 months for germination, so you must be very patient.
On the other hand, a few of the species types can sometimes be very quick to germinate.
Suzy

I have seen clematis seeds sold at Wal-Mart last summer. Check them out. I bought my clematis (Texenis?) potted from Trader Joe's last summer. They don't carry it all the time. You might ask them when clematis will be available there again. Hope this helps. If all fails you can order from Chalk Gardens in Northern Calif.

Sorry, but the name to order clematis from in Northern Calif is Chalk Hill, not Chalk Gardens.

Malibu Rose,
Clematis Texensis is a species type which is so rare to find in the U.S. that I have had to collect Texensis seed from breeders in Europe. I would imagine that your plant is likely a popular hybrid of a Texensis cross, such as Duchess of Albany or Gravetye Beauty, Princess Diana, etc...
Suzy