I used to recommend the Broadcast Warehouse 1 watt exciter card, pro quality at a decent price. I don't know the status of it now, but you can do a search and find out.
One that I hear a lot about these days is EDM transmitters, which seem to be most popular for Christmas displays, which is how I found out about them. The proprietors of those displays are quality conscious, and a lot of those guys seem to be sold on EDM. I haven't tried EDM, but it seems real people are saying they're good.
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www.edmdesign.com]
Up to 100 milliwatts, can be set for Part-15 output power levels as well. Site includes free sales language and testimonials!