It may be cheaper and more convenient to commission something like this from a handy friend or local carpenter. You would have to pay for his time designing/building it, of course, and the lumber. But that way, they can build it in
your house. So you don't have to worry about shipping or assembling it yourself or trying to fit a pre-built unit through your door.
That said, I found this loft
online (there may be other companies who offer such a service, but this is the first I found). They say the frame capacity is 1000 lbs and the mattress support can hold up to 400 pounds (with an option to add 200 more pounds of capacity with reinforced slats, so 600 in total). I would think you and your partner fit that requirement, since a couple on the heavier side of that probably wouldn't willingly climb a loft every night in the first place.
I don't know your price point, of course. How expensive is "affordable" for you? The prices on that site are: $268 for a queen loft (that you assemble yourself). $105 for shipping. If you want additional weight capacity above 400 lbs, it's $25-$75 depending how much more you need. I couldn't price their king-sized version, you have to call for that. It doesn't seem like a terrible price, considering how much a good bedframe can run these days. But that's a lot in shipping (another argument for localsourcing). You might want to get the slats even if you and your partner aren't very heavy, depending how strenuous your, uh... activities will be. Also, a loft is going to cut off any sources of light from the ceiling or upper windows (if that's a concern). Just some things to think about!
What are you looking to put underneath it, by the way?