You can find used RG7420's and RG7620's on eBay for pretty cheap often, but production models now, the cheapest are the S7320 and the RG1527 which run for around $500-$1000 depending on where you live. So I'd say eBay would be your best bet for finding an Ibanez 7 string with a trem.
I dont recommend buying a low price guitar that comes with a trem, because theyre usually garbage