I recently purchased a used Rollie medium format slide projector from Leos cameras in Vancouver, BC, Canada for $200 CDN.
Anyone familiar with Leos' pricing strategy will understand why I remain amazed at this accomplishment
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Anyways, it still is possible to get used medium format projectors at reasonable(ish) prices.
It is also reasonably easy to get slide mounts. There are a couple of sources for Gepe mounts here in Vancouver. Glazer's in Seattle had them on the shelf a couple of weekends ago, as well.
I also bought a Gepe slide mounting accessory last year, which I haven't yet worked with.
What I cannot direct you to is a lab that will both develop and mount medium format transparencies. I expect that there may not be anyone who does this any more.
6x6 and 6x4.5 is definitely workable. The 6x7 slide projectors are mythical beasts (where the myth factor relates to the budget required).
Matt